First thing I'd do is backup the library as it is now, to another device or the cloud.
Did you check the drive - is it full, do the h/w manufacturer diagnostics report any problems?
Try this - copy the library in its entirety to another drive, and use that instance of the library - what happens when you do an Add? If you still have problems I suspect the database is corrupt - try dong a Library Maintenance->Restore database..
If you can add then its probably the other drive. Maybe the the Ext directory tables are corrupt, I assume Kubuntu has fdsk try running it to see if there any errors.
BR
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