That message can have various causes.
To name a few:
1) That a disk does not have enough space or is no longer accessible.
2) It is a corrupt archive-in-archive.
3) You have put the extension in auto delete.
4) The files are no longer complete (partly deleted) at your newsserver.
5) A ‘settings’ database file has become corrupted.
There are many more like that.
To rule out a few things, you might first start by setting Usenapp to the factory defaults. If you make a backup of these files, you can restore them later if necessary.
The tutorial to accomplish this can be found at:
https://www.usenapp.com/forum/d/324-reset-usenapp-to-default-settings