First occurence:

14:52:52   6.00M   rootpool/ROOT/default@2019-11-19-13:53:43-0
sdhci_bcm0-slot0: Controller timeout
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x01e86000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000030
sdhci_bcm0-slot0: Argument: 0x00a1f8e8 | Trn mode: 0x00000026
sdhci_bcm0-slot0: Present:  0x01ff0506 | Host ctl: 0x00000003
sdhci_bcm0-slot0: Power:ESC[4C0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:ESC[4C0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000000
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff00bb
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:ESC[5C0x00000000 | Caps2:ESC[4C0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================
14:52:53   6.41M   rootpool/ROOT/default@2019-11-19-13:53:43-0

After this message the machine freezes, e.g. even the serial console hung.
Had to toggle the power.

Verification run. More and different console messages.

23:32:26    357M   rootpool/ROOT/default@2019-11-19-13:53:43-0
sdhci_bcm0-slot0: Controller timeout
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x01f1f000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000006
sdhci_bcm0-slot0: Argument: 0x012775c0 | Trn mode: 0x00000036
sdhci_bcm0-slot0: Present:  0x01ff0a06 | Host ctl: 0x00000003
sdhci_bcm0-slot0: Power:    0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:    0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000021
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff008a
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:     0x00000000 | Caps2:    0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================
sdhci_bcm0-slot0: Got data interrupt 0x00600000, but there is no active command.
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x00000000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000010
sdhci_bcm0-slot0: Argument: 0x03a19610 | Trn mode: 0x00000036
sdhci_bcm0-slot0: Present:  0x010f0000 | Host ctl: 0x00000002
sdhci_bcm0-slot0: Power:    0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:    0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000000
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff008b
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:     0x00000000 | Caps2:    0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================
sdhci_bcm0-slot0: Got data interrupt 0x00600000, but there is no active command.
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x00000000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000010
sdhci_bcm0-slot0: Argument: 0x00019a10 | Trn mode: 0x00000036
sdhci_bcm0-slot0: Present:  0x010f0000 | Host ctl: 0x00000003
sdhci_bcm0-slot0: Power:    0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:    0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000000
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff008b
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:     0x00000000 | Caps2:    0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================
23:32:27    359M   rootpool/ROOT/default@2019-11-19-13:53:43-0
23:32:28    361M   rootpool/ROOT/default@2019-11-19-13:53:43-0
sdhci_bcm0-slot0: Got data interrupt 0x00200000, but there is no active command.
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x00000000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000010
sdhci_bcm0-slot0: Argument: 0x00019a10 | Trn mode: 0x00000036
sdhci_bcm0-slot0: Present:  0x01ff0000 | Host ctl: 0x00000003
sdhci_bcm0-slot0: Power:    0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:    0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000000
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff008b
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:     0x00000000 | Caps2:    0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================
sdhci_bcm0-slot0: Got data interrupt 0x00200000, but there is no active command.
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x00000000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000010
sdhci_bcm0-slot0: Argument: 0x00019800 | Trn mode: 0x00000036
sdhci_bcm0-slot0: Present:  0x01ff0000 | Host ctl: 0x00000003
sdhci_bcm0-slot0: Power:    0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:    0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000000
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff008b
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:     0x00000000 | Caps2:    0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================
sdhci_bcm0-slot0: Got data interrupt 0x00200000, but there is no active command.
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x00000000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000010
sdhci_bcm0-slot0: Argument: 0x00019a00 | Trn mode: 0x00000036
sdhci_bcm0-slot0: Present:  0x01ff0000 | Host ctl: 0x00000002
sdhci_bcm0-slot0: Power:    0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:    0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000000
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff008b
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:     0x00000000 | Caps2:    0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================
sdhci_bcm0-slot0: Got data interrupt 0x00200000, but there is no active command.
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x00000000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000001
sdhci_bcm0-slot0: Argument: 0x00019802 | Trn mode: 0x00000012
sdhci_bcm0-slot0: Present:  0x01ff0000 | Host ctl: 0x00000002
sdhci_bcm0-slot0: Power:    0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:    0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000000
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff008b
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:     0x00000000 | Caps2:    0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================
sdhci_bcm0-slot0: Got data interrupt 0x00200000, but there is no active command.
sdhci_bcm0-slot0: ============== REGISTER DUMP ==============
sdhci_bcm0-slot0: Sys addr: 0x00000000 | Version:  0x00009902
sdhci_bcm0-slot0: Blk size: 0x00000200 | Blk cnt:  0x00000010
sdhci_bcm0-slot0: Argument: 0x00019a00 | Trn mode: 0x00000036
sdhci_bcm0-slot0: Present:  0x01ff0000 | Host ctl: 0x00000002
sdhci_bcm0-slot0: Power:    0x0000000f | Blk gap:  0x00000000
sdhci_bcm0-slot0: Wake-up:  0x00000000 | Clock:    0x00000207
sdhci_bcm0-slot0: Timeout:  0x0000000e | Int stat: 0x00000000
sdhci_bcm0-slot0: Int enab: 0x01ff00bb | Sig enab: 0x01ff008b
sdhci_bcm0-slot0: AC12 err: 0x00000000 | Host ctl2:0x00000000
sdhci_bcm0-slot0: Caps:     0x00000000 | Caps2:    0x00000000
sdhci_bcm0-slot0: Max curr: 0x00000001 | ADMA err: 0x00000000
sdhci_bcm0-slot0: ADMA addr:0x00000000 | Slot int: 0x00000000
sdhci_bcm0-slot0: ===========================================

Had to toggle the power as well.