Thanks to a fellow user of the forum, a workaround has been found to
avoid the error. Set Option "NoOutputInitialSize" "1640 1312" in the
xorg.conf's device section to allocate more memory to the X server, and
also set cma=512m in the kernels command line to the GPU.

The question remains why this was not necessary under Chimaera (maybe a
kernel issue) and using xlibre on the exact same freia installation.

Nevertheless I do not think that this behaviour is originated in
Devuan's own xserver packaging. If there has been a change since the X
version on Chimaera and also the point where xlibre was forked, it has
probably made upstream.

So from my side the issue may be marked as solved.