FreeBSD version 10 and earlier - VMware Workstation 16 Player (Non-commercial use only) Player root@:/mnt # cat /var/log/messages ... Nov 19 21:13:53 mountd[4634]: bad exports list line mnt/b/
https://man.freebsd.org/cgi/man.cgi?query=exports&apropos=0&sektion=5&manpath=FreeBSD+10.2-RELEASE&arch=default&format=ascii --- ... ... The first is to list all mount points as absolute directory paths separated by whitespace. ... ... The second is to specify the pathname of the root of the file system followed by the -alldirs flag; this form allows the host(s) to mount at any point within the file system, including regular files if the -r op- tion is used on mountd(8). ... ... ... The third form has the string ``V4:'' followed by a single absolute path name, to specify the NFSv4 tree root. ... ...
Obviously / apparently the 2nd form requires also absolute path. Not sure why the absolute path is explicitly noted for the 1st and 3rd form and not for the 2nd...