It is possible that my webhosting provider changed something
Most likely this is the reason.
Make sure first that the regular mail function is working. Run a simple script like this below, substituting your own email address, and check if it really works.
mail('your_email_here@your_domain', 'subject test', 'body test');
Depending on your result, you have to investigate it further. If even the simple default mail() function of PHP doesn't work, surely miniBB extended function will not work, too.
Running in Safe Mode also is not perfect. You may ask your provider what is the reason of turning this option on, and is it possible to change it. In Safe Mode, sending emails may also be disabled.
Should I try to remove this fifth parameter from mail function whenever it is used?
This may help. It's in bb_functions.php -> sendMail() ->
if(defined('EMAIL_NO_FLAGS')) $noFlag='-r'.$errors_email; else $noFlag='';
mail($email, $subject, $msg, $from_email, $noFlag);
try to remove $noFlag
in mail() function.
Also, ask your provider what MTA is used for sending emails. Is it Postfix, or Qmail or Sendmail?
Let us know the results. I am working on a new miniBB version where sendMail() function will be a bit improved regarding meeting RFC compatibility and Postfix. I could provide you updated functions, so you may try them out, may be this will help. You know my email :)