Re-uploading just the one file is certainly easy enough to check and rule out.
You might also want to review the installed extensions, for whether one or more of them hasn't actually been updated for phpBB 3.3.x compatibility. Or, that the updated version which is compatible with phpBB 3.3.x has not yet been installed on your board.
Because even if you have all the right phpBB files uploaded, it can actually be an extension which still expected to use
, which no longer exists in phpBB 3.3.0 and later. OneAll Social Login was one such extension
a couple people here encountered, but certainly not the only extension which could be trying to perform registration-related actions which presumed
must still be handled.