That error message popped up just now as I was testing a new set of OSC strip mute commands,
I’m installing Ardour 6.5.0 on Debian 11 to run these.
I restarted the box and the error persisted through restart, launching Ardour and opening a fresh session. As soon at the strip mute OSC command hits, the session issues the error and exits.
I found a conversation on the Ardour Tracker back in 2008 that didn’t really provide many clues as to what’s going on (0002245: critical: event pool out of memory - recompile with larger size!! - MantisBT). I could follow the instructions there, or I could leave it broken on this machine if having a live/broken installation would be useful for debugging.