.xsession-errors file has many statements like 'fvwm-menu-desktop: XML::Parser not installed.'. Installing p5-XML-Parser seems to fix this. Seems that fvwm2 needs that in runtime so maybe its good to be a dependency for fvwm.
A commit references this bug: Author: cy Date: Thu Jun 23 03:01:29 UTC 2016 New revision: 417340 URL: https://svnweb.freebsd.org/changeset/ports/417340 Log: Register a textproc/p5-XML-Parser runtime dependency for fvwm-menu-desktop(1). PR: 210478 Changes: head/x11-wm/fvwm2/Makefile
Fixed.