Most of the time these errors are related to pictrs not responding fast enough. Usually because it has to convert the picture, video, gif and pictrs is already busy.
If you ever get these messages try again after 30 seconds (or so) and it should be fine
There is a correlation with file size. The picture I tried to upload was particularly big - about 20MB I think - so I probably should have downsized it anyway. There was no problem with uploading the few KB big screenshot.
20MB is the max size so it's probably an issue with file size rather than a timeout or something along those lines. It's interesting that our webserver didn't block the connection before adding it to pictrs.
If it works with a smaller image please let us know.