I’ve experienced this issue since the recent sever update to 1.28.2.6106.
It only happens to my movie library, not TV libraries.
Both app.plex.tv and local IP returned the same results.
I’ve also tried clearing browser caches with no luck.
But switching to other users doesn’t have this issue (only happen to the main admin user).
The URL does change to “pivot=library” or “pivot=collections”.
I think someone had this issue earlier this year. Seems to be an issue in the configuration of the web client. Was fixed by resetting the web client back to default. Note, that means you need to reconfigure the web client as if you just logged from the web for the first time. The server itself still has all the metadata and videos, and your account remembers what you watched. But the customizations to the client - such as pinned libraries and what order they are in - will be lost.