Please e-mail me the log file:
so that I can see more about how this error happened. This looks like an error caused by a bug that I recently fixed for the next release, but looking at the log I’ll be able to tell for sure.
No, the 0 is an internal error code that has to do with which subsystem of EagleFiler the generated the error.
Yes, you should quit and restart. You will lose some data—if there were nothing to save, EagleFiler wouldn’t be trying to save the library—but almost certainly nothing of consequence. EagleFiler auto-saves the library quite frequently, so the amount of unsaved data is likely small. For example, if you were importing a bunch of files, the last-imported file might have been copied into the library folder, but (due to the saving error) not recorded in EagleFiler’s database. If you see a file missing when you re-launch EagleFiler you can simply re-import that file.
In this case, the tagging has probably not been saved into the database, but you see it in EagleFiler’s interface because the changes have been made in RAM.