Problems after changing shop domain to www

[css@evo-con]

I have the shop on the main domain, but want to change it to the www subdomain. After the change I keep getting logged out from the back office, for example every time I try to clear the cache or save the maintenance mode page. I also get logged out at random times on other clicks.

Another problem is that the front office isn't visible in maintenance mode (I get the maintenance page), even though my IP address is set in the list of maintenance IPs.

After returning to the main domain these problems disappear again.

The shop version is 1.7.6.1. An update is currently not possible because we had to modify some core files. But none of these modifications touch the shop domain and they don't make any problems on the main domain.

Can anyone give me a hint what might cause these problems?

Thanks in advance,

Christian Schmitt-Schwehla