@lmense file has been received – no firm conclusion yet, some local filesystem / sandboxing system hiccup local to your system is still my top suspect as we're unable to reproduce it as of yet and that's what the logs seem to be suggesting too. I should also note that that warning dialog is quite noisy and is not reflecting the incapacity to save the document altogether. You should not be encountering any form of data loss (it's basically one autosave iteration failing, which will succeed on the next attempt and on hitting File > Save manually, and it's also automatically done on closing the document).
I do have a possible workaround in mind – does clearing the cache directory created by Manuscripts application help? If it does, then it is indeed an issue technically with macOS and it somehow having got the local filesystem in a bad state. To clear the Manuscripts cache directory, please complete the following steps:
- Open Finder.
- Go > Go To Folder (or hit Shift+Command+G)
- Enter ~/Library/Containers into the window that opened and then click "Go"
Once you've done that your Finder window should be now in a folder where you see the folder "com.manuscripts.Manuscripts".
- Please drag the folder com.manuscripts.Manuscripts into Trash (this clears the cache directory that contains safely deletable information that improves the performance of Manuscripts and which it uses when exporting documents, etc).
CEO & Co-founder of Manuscripts.app Ltd