I ran into an issue today, so I will post the solution in case it helps someone else.
Joplin server sync stopped working, with the following error message:
2023-03-14 22:10:51: [error] App: 10.0.1.254 TypeError: Cannot read properties of undefined (reading 'content_size')
at ItemModel.<anonymous> (/home/joplin/packages/server/src/models/ItemModel.ts:276:57)
at Generator.next (<anonymous>)
at fulfilled (/home/joplin/packages/server/dist/models/ItemModel.js:5:58)
at processTicksAndRejections (node:internal/process/task_queues:95:5)
Also the following warnings were logged (hundreds per second):
2023-03-14 22:07:43: [warn] ShareModel: Found an item (7D9zXuCk5oEkhHFXAzDebWIQijEpk6hN) associated with a share that no longer exists () - skipping it
2023-03-14 22:07:43: [warn] ShareModel: Found an item (3g4jHUb1OGbnzqPV34sjmnINNlUF0vEP) associated with a share that no longer exists () - skipping it
It turns out the PostgreSQL indices were corrupt. To fix this, first get a postgres shell:
docker exec -it ... psql -U <user>
Then rebuild indices:
REINDEX DATABASE <database name>;