Sometimes, clearing out all the files saved on your Prepd Offline extension and doing a full synchronization can help fix any corrupted data which may cause issues like: never-ending sync, blank screen after sync, inability to open folders/articles, and others.
To do a data wipe, please click on the "Wipe Data" button from the Prepd Offline Launch page and, then, Synchronize as usual.
If the issue persists after the wipe, you might want to consider a re-installation of the app (click here to learn how).