I get this error

Warning [PHP]: Cannot modify header information - headers already sent by (output started at modules/net2ftp/language/english/modinfo.php:40) in file modules/agendax/index.php line 726

when I try to access Agenda-X. Actually it started by telling me that the camportal module already sent the headers so I deactivated that. Then it told me it was the guestbook module so I deactivated that. Now it's telling me this. Any ideas on why this keeps happening?

been editing files through a cPanel interface? It is reported that this may give additional linebreaks after the closing ?> in the language files.

In this case it is the modules/net2ftp/language/english/modinfo.php that (most likely) has linebreaks after the last ?>

Check the file to see if this is the case. If so, remove them so ?> is the very last of the file

No I don't use the cpanel interface for that. I have heard that's not good. I edited the line breaks as you suggested and all is well. Thank you for your quick reply!