Of course, still no effect
EDIT: changes to other fields apply correctly
Viewing post in Can't replace project cover image once set
Sorry from me as well, I solved it, and the reason was pretty easy, though it might still leave some space for improvement in the website UI...
When using a slow connection to upload a GIF, it might take a while (and with "a while" I mean even several minutes) for the new image to be displayed and, as apperently it only gets saved right after displaying, if someone reloads the page or tries to upload it again in the meantime as the website gives no feedback for the upload process it will break everything and stop the upload.
One quick solution I would suggest is to improve the visual feedback on the image upload so that the user knows that things are still going on :)