Thanks a lot! I tried making an edit from the line where there was an editing error, and on my side it did work:
What I notice is that this edit is the exact inverse of the previous edit on this item, meaning that it's labeled as a "Manual revert" in the history:
https://www.wikidata.org/w/index.php?title=Q128586674&action=history
Therefore I wonder if you might have been prevented from doing this by an "Abuse Filter" which prevents certain types of users from doing certain types of edits. It could be that my account belongs to different user groups, meaning that I don't get blocked by the same filter.
It could also be that the servers were under a heavier load when you tried and that the edit would work again on your side.
One way to test the abuse filter theory would be to craft an edit which is knowingly violating some known filter and checking what the error message looks like in that case - I wouldn't be surprised if it was also "The save has failed".