Do you use an external provider for uploads (amazon s3) ?
If not, I have two guesses (but I’m usually bad at this): you added some css custom for cooked images and it affects uploads also you can try /safe-mode, deactivate plugins and theme et check if it works.
Or during the post processus, the forum generates an optimized image and it doesn’t work. You can try deactivating the create thumbnails setting and upload a new image (you can’t use the same, because of a caching magic that remembers this image). It will not create an optimized version and it might work.
Check the /logs page anyway, there might be some useful info