I had some old Ardour2 projects which I could not access with Ardour5. I could not find working Ardour2/Ardour3 binaries for Mint 19 so I had to compile Ardour3 and fix multiple build issues. In case somebody else finds this useful changes are here: https://github.com/Olzaq/ardour/commit/a40fa7a3f5d23b7e82f979fcaf64e4d66be8f69b
Try Ardour 6. That version features various improvements in backwards compatibility, and is known to load many Ardour 2 sessions that failed with earlier versions.
If it still does not load, it would be great if you could share the session file so that we can fix backward compatibility. Ideally at tracker.ardour.org