I just posted this to OllisGit:
I spun up a clean instant of Octoprint and installed postgresql on it after updating everything on the instance except updating to Python3. It's running Python2.7. I'm running that as a standalone Octprint instance not attached to a printer, hosting the external filament database only.
I'm running two Octoprint instances, one attached to a Qidi X-One2 and a second attached to an Ender 3 Pro. The Qidi octoprint has not been updated to Python3. It immediately attached to the new external database.
The Ender 3 Pro octoprint was upgraded to Python3, and was the original external database server. It would not attached to the new external database server, and the non Python3 octoprint could not attach to it.
I spun up another new instant of octoprint for the Ender 3 Pro. I updated everything on it except updating to Phython3. It's running Python2.7. It immediately connected to the new external database server.
To summarize what I figured out is a client running Phython3 will not connect to an external database server period. If the database server is running Python3, no clients can connect to it regardless of what version of Python they are running.
For a connection to an external database server to work neither the client or server can be running Python3.
The issue may be in the configuration of Filament Manager under Python3, but if so, I'm not smart enough to figure out what the issue is.