I saw two VE data loss save failures this weekend. In each case, the user hit "save page" and the dialog just froze forever. The data was not written, and the "resume editing" button no longer worked, so data was lost (hence high priority).
If it is helpful for debugging, the first time was by enwiki user Cindyshih around 00:00 UTC Mar. 7/8 in her personal namespace (apparently she never came back to finish/retry the edit, so I don't know the exact name of the page she was editing), and I then saw it again on https://en.wikipedia.org/w/index.php?title=Bessie_Harvey about 30 minutes ago.