If all other pages working, you must check changes you did. You can't check all files, because too much, but problem is 100% in files you worked/edited.
Here can be php error or more possible, as said previous, this is problem with whitespace or dom characters before <? in file.
Edit file where error is showing with Hex Editor XVI32 (easy to find on the web). In right window, first two signs must be <?
If is empty space or strange characters, just delete it. Should help