and no change.

You need to disable all kind of errors non only php. 

line 595:

 $this->output();

and commented out

//$this->output();

and see again http://yoursitename.com/base/captcha/ in firefox. 

still does not want to display errors

But one thing I found: 

I compare in Chrome  

OxW-site:

-> view-source:http://www.oxwall.org/base/captcha/

AND my site:

-> view-source:http://www.my.site/base/captcha/

it looks like a encoding (charset) error or something like that...

after oxwall upgrading from 1.7.0 to 1.7.1 was added  new button "MORE" in profile,  I could not translate it in administration so I translated it in code "/web/ow_system_plugins/base/components/profile_action_toolbar.php" and probably I changed the encoding of this file and therefore captcha did not work...

Thank you  ross for your patience and effort to help :)