I get this error when switching between Projects that have MIDI tracks, but the instruments are different.
For instance, Project A has track 5 as Sitar while Project B has track 5 as Bassoon.
Not sure it's related, but I have multiple soundfonts installed.
PCG files are pre-computed graphics files for audio clips, so the MIDI tracks shouldn't matter. Do you have audio tracks in the projects?
Yes, both projects have a mix of midi and audio.
Are the project files you are switching between from the same project 'directory'?
No, each project is in it's own directory (inside the Projects folder). I do have a couple versions of each project in their respective folders, but only notice the problem switching projects (old versions are kind of a back-up)
What you can do is delete all .pcg files in the project's Samples/AE5PCGfiles directory. They are recreated automatically so it's harmless. It would be better if you could zip the two projects and send them to us, but I reckon that would be a massive amount of data.