php post_max_size: 8 MB
upload_max_filesize: 2 MB
The user starts the upload process, after some time the file upload to the server is completed (both xenforo and php allows for 8 MB post uploads). I could see in chrome the progress achieved 100%.
Then there is an error message (The file was too large to process).
My suggestion is to log that error in acp (log upload errors when upload size < xenforo max file size) so the admin is informed his setup does not work.