Most likely the metadata agent of that library got changed to “Personal media” at some point in the past.
Try changing it back using the hosted web app.
Make sure your web browser doesn’t have any browser add-ins activated which could interfere.
Additionally, verify that your server’s database file is in good health.
Or inspect them yourself. Take a look at the Plex Media Server.log file and seek for messages about database corrupt or malformed.
If you find these, you may have to repair your database. https://support.plex.tv/articles/repair-a-corrupted-database/
That change is a direct result from selecting “Personal Video”/“Personal Media” as the agent.
An “Other Videos” library is actually a Movies library, just with a different agent.
Probably because the change is also a change in metadata agent “generations”.
“Plex Movie” is the new world, whereas Personal Media is the old world.
You are not the first where changing between them is a one-way road.