We are using listbingo for ads and also CB for users login etc. We have a button in our homepage in order for a user to start posting an ad.
After we enabled joomsef we are experiencing the following problem: If the user is already logged-in there is no problem. If the user is not logged-in and hits the button to post an, ad instead of returning just a message that you have to login first to post, it returns a "JoomSEF Error - Headers already sent" page. If you press the back button of your browser it then shows the correct page, the one with the message "Login to post".

Hi,
The "Headers already sent" error is caused by some output being generated by some plugin before JoomSEF could parse current URL, so the headers are already sent and JoomSEF can't send correct header code according to the parsing result.
Does the button to post an ad use standard listbingo functionality, or is it custom implemented?
Check your server error logs if there's any error logged when the page with "Headers already sent" message is displayed.

The address bar - after the error- shows www.tradeordonate.com/component/comprofi...&time=1332358763 so I was thinking if there is a problem with cb login and artio joomsef... Because as I was saying it does that only if you are not logged. If you are logged-in everything is ok. Mayby something with cb login...

The button is a standard functionality, the "post ad" feature of listbingo.

P.S. I am a newbie so how can I check my server logs? Should I ask my hosting provider, or is it something I can do from Joomla's backend or cpanel?

I found the below errors just after I pressed the buttton to post an ad:

[Wed Mar 21 21:55:56 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/404.shtml, referer: www.tradeordonate.com/

[Wed Mar 21 21:55:56 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/components/com_listbingo/templates/adflyer/images/titleborder.jpg, referer: www.tradeordonate.com/

[Wed Mar 21 21:55:56 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/404.shtml, referer: www.tradeordonate.com/

[Wed Mar 21 21:55:56 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/components/com_listbingo/templates/adflyer/images/shadowbox.jpg, referer: www.tradeordonate.com/

[Wed Mar 21 21:55:51 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/404.shtml, referer: www.tradeordonate.com/

[Wed Mar 21 21:55:51 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/components/com_listbingo/templates/adflyer/images/titleborder.jpg, referer: www.tradeordonate.com/

[Wed Mar 21 21:55:51 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/404.shtml, referer: www.tradeordonate.com/

[Wed Mar 21 21:55:51 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/components/com_listbingo/templates/adflyer/images/shadowbox.jpg, referer: www.tradeordonate.com/

[Wed Mar 21 21:55:20 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/404.shtml

[Wed Mar 21 21:55:20 2012] [error] [client 46.177.87.93] File does not exist: /home/doros/public_html/favicon.ico

The errors you've posted are probably not causing this error. We'll test CB login with JoomSEF and let you know.
What versions of Joomla, JoomSEF and CB do you use? Do you also have our CB extension for JoomSEF installed?

Joomla 1.5.23
JoomSEF 3.9.5
CB 1.8
I have the JoomSEF for Listbingo addon and also CB extension for JoomSEF v.2.0.3.

Regards