Comment 5 for bug 1596382

See above. "Autosave uses another path than save, and points to the Bin folder by default"
- and of course writes to Bin fail.
This occurs when File Append is used, not file-open, so there is no active/alternative file-path defined at this stage, but auto-save tries to work on what is loaded.

ie it looks like missing or poor default choice for Autosave path, when no other information is there.