by Davo Smith.
Have you installed my plugin or are you using the standard version built-in form Moodle 2.6 onwards?
Whilst the built-in version was based on my code, it was significantly rewritten and so you should not take any notice of documentation written for my plugin, if you're not directly using it.
It sounds like the error message is what you need to investigate - turn on Debugging and see if you can find out a bit more information.