SUMMARY
In 5.0.0 39f51c6, if you try to save with 'Save Incremental Version' or 'Save Incremental Backup', the operation does not save and produces an error. It fails both for local storage and Google Drive. A normal document produces a different error than an autosave file open on startup after a crash. 

STEPS TO REPRODUCE
1. Open a document 
2. Make some changes
3. Save with 'File' -> 'Save Incremental Version' or 'Save Incremental Backup'

OBSERVED RESULT
The document is not saved. Saving a normal document says: "Cannot open file for writing. An unspecified error occured.".

Saving an autosave produces either a popup with "Couldn't save incremental version. Alternative names exhausted, try manually saving with a higher number." (for incremental version), or an error in the status bar: "Error during saving: / cannot be written to. Please save under a different name." (incremental backup).


EXPECTED RESULT
The document is saved.