This happens regardless of whether it is an already uploaded file or not. Uploads in posts work just fine. I updated over the weekend. Same in safe mode.
I can repro this in a clean install, but not here on meta.discourse.org (nor on Stable).
Hmmm currently I’m not able to reproduce this myself. I tried locally with my developer setup and on a self-hosted site I just updated and I’m able to change the profile picture without any errors.
Meta is also running the latest so it should be reproducible there as well.
I know you tested in safe mode, but maybe there is still a plugin or something interfering?
Have you tried it without the sidecar plugin? That seems to be the only unofficial one. Maybe the new image processing for YouTube thumbnails interferes with the upload of profile pictures.