I don't have this problem in joomla 1.7 and MarvikShop 2.0
What version of Joomla and marvikshop do you use?
I found a solution on this site:
Documentation- FAQ- Login problem administrator
Hope this helps you.
When you have a login problem, this might be a symptom related to Joomla 1.5.16 or higher. They have tried to increase the security and introduce a bug that is reported in the bug tracker.
If you have a joomla 1.5.16 of higher, perhaps you should have rollback the sources
/libraries/joomla/application/application.php or put the session line in comment at line ~530.
Joomla bug tracker
$session = &JFactory::getSession();
// we fork the session to prevent session fixation issues