ok, I now found some threads about this (didn’t find them with search before, sorry):
Nextcloud 18 - Problems with Community document server and OnlyOffice
mentioned problems appearing on Linux servers:
Document server fails with “./x2t: Permission denied (github)”
Unable to use community document server due to issues executing x2t (github)
and some small solutions in here:
OnlyOffice / Document Server does not work
I will follow them and wait before I update…