To workaround, I have started waiting for the media browser to finish loading, then I count to three, and it usually works with the three seconds, but not always.
Yesterday I had an upload fail. The error made it sound like it was impossible to upload images to said community. I waited ten seconds, tried again, and it worked.
Probably lemmy.world servers were struggling. If it keeps on happening make an alt on something that accepts images straight away (I think slrpnk.net does) and test there via voyager.
Had this issue. Happens to me when using the image selector that pops up. However, it works fine if I access the files and select the image from there.