When clicking "Save", especially after leaving an unsaved document open for a while (Linux/Firefox), the button is stuck loading forever, and nothing seems to happen. Refreshing+restoring+saving again does not seem to help. This has happened multiple times now.
The only workaround I have found is to copy-paste the working document in another Gitbook workspace, discard the changes, start a new draft, paste the changes, and save again. This usually fixes the problem, but is not very ergonomic.