[SOLVED] Checking if username is available don't stop loading

[Divvy]

Hello guys,

I have an issue in my registration page...
When I write a username and a captcha, will show this:
"Checking if username is available"
And don't stop...



I disabled all my plugins and the problem continues...

I know that I can disable AJAX: ACP > Configurations > Server & Optimization Options > Enable XMLHttp request features? > to No

But that is not a solution for me... I really want the XMLHttp request feature enable.

Please help me... since that is not a plugin problem...
What could be?

My website: http://www.vozesnaweb.com

Thank you!

P.S. I created a topic in mybb.com a few weeks ago and didn't received any help