sorry for being silent so long after asking for help.
I found a solution for my issue with NSM sessions created with Ardour 5, see this:
I found that in my original session.nsm files Ardour was referenced with the full path, e.g.:
Linking an Ardour 6 version to /usr/bin/ardour5 doesn’t work, neither does changing that line to
with /usr/bin/ardour being the Ardour6 binary or /usr/bin/ardour6 being a link to the Ardour6 binary, see the original discussion.
What does work is not using the full path in the session.nsm file, i.e.:
Hope this helps if someone else stumbles upon the same issue.