Hi Mats,
Looks like a bug in Event Calendar, it is not correctly handling the url because it does not have .aspx extension.
When you click the next month button it makes an url like this:
http://haningebudoklubb.se/kalender&visdate121=2010-10-01T00:00:00
but that is not correct, it should be
http://haningebudoklubb.se/kalender?visdate121=2010-10-01T00:00:00
I will fix this for the next release, for now the only workaround is to change the url to kalender.aspx instead of kalender
Hope it helps,
Joe