Thanks @Slabrie.
Finally found it. But was still different.
There were no old or stale files on my Linux machine. I checked every .cfg, cache, project folder.
However, on my Mac, there was an older version of 2025.2.2 still installed. And it had three framestore partitions configured, two of which still existed, and one which was on a disk that is no longer with us.
After removing the three framestore partitions from the sw config on the Mac, uninstalling the old 2025.2.2 version of Flame and deleting all old projects there, now on my Linux Flame it starts up quickly and I no longer see that error message.
So the Linux Flame somehow communicated with s+w on the Mac, even though Flame was not running there, and hadn’t been since the last reboot. Saw those frame stores, but couldn’t communicate with them. I guess s+w was alive enough to share config, but not alive enough to answer appropriately.
Problem solved on my end. But it seems like there’s some gremlin in how this behaves. That was multiple hops of debugging, and it’s not uncommon to have old stuff somewhere that has been long forgotten.