There are certain Calf plugins, inserted in old sessions (Ardour 5), that Ardour 5 and 6 is not able to insert. The problem is in the unique-id of these plugins that has changed.
To solve the problem I have edited the session with a text editor and I made the following changes, for example:
It doesn’t sound like this is a problem in Ardour, that is exactly what is intended to happen I think. The problem is that in upgrading the plugin Calf broke existing sessions by changing the plugin to be an entirely new plugin as far as Ardour (And any host) would be concerned.
oh, yeah I had to reinstall my OS and had the same problem after reinstalling the CALF plugins… Luckily for some of them I had the presets saved, which were working. At least some of them.
I also had the same problem with dragonfly reverb and helm…