From what it seems online, it would not be due to any flash units, but rather the hotshoe on the flash itself, and components of it causing the camera to think that there's a flash on it and causing the pop-up flash to be 'stuck'. Error 05 comes out after 2-3 tries of pushing the pop-up flash button.

The recommended reading would be from the following links:
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=000xmg
http://photography-on-the.net/forum/...d.php?t=203699