Foren / Cotonti / Support / function.mb-stripos is causing an error

We enabled mod_deflate.c on our server and since than if people visit our website from an external link this error shows on top of the page:

Warning: mb_stripos() [function.mb-stripos]: Empty delimiter in /home/armaholi/public_html/testers/armaholictest/system/common.php on line 483

I have been breaking this down for the last few hours as this error does not show up when I use a fresh cotonti databse on the testsite and visit it from an external link. However, if I use our current databse on the test site and visit through an external link the above error pops up.

I can fix the error by commenting this line in the common.php:

&& mb_stripos($sys['referer'], $cfg['hostip']) === false

(line 483)
This will fix the issue in our current database but I am not convinced thats a real fix. I wonder why on a fresh database this is no issue at all while on our current and active databse this is causing an error.

Just check for yourself: http://www.armaholic.com/testers/armaholictest/
This will show you a bugged page (layout) with the error mentioned on top. When you however browse the site all is fine. Its just cause of some weird error with the referrer.

I have tried updating to 0.6.13, did not help.
I have cleared the DB (logs, trash, referrers etc), did not fix it.
I have cleared all custom tables from the DB (from plugins) to bring it back to only the default tables but that too did not help.
The only thing which works is commenting line 483 in common.php.

Do you guys have any idea whats going on?