Forum / Helpdesk - domain problem

first of all i am using:
TYPOlight webCMS 2.7.2
and Forum / Helpdesk v1.0.0 stable

i try to add a "public support" category,
i check reply only, so ppl can only reply to my created threads/tickets
Frontend URL base (comment says: Should be something like http://www.example.com/forum)
i put in: http://lanparty4.us.tt/index.php/Forum
and all links do NOT work (i already found out why, but i cant find a solution to fix)

if i change the base url to:
http://lanparty4us.la.funpic.de/index.php/Forum
everything is working fine... but actually BOTH are pointing to exactly the same directory?!
any ideas why and/or how to fix this (i do not want to show up my webhost...)?