Multisite after server migration: wp-admin comes to error 324

Hi at all,

today I transfered my wordpress multisite with the whole database to another server (same OS). The single sites run as they should. But if I try to login via /wp-admin or wp-login.php I get the error: empty response at my browser. I can´t reach the backend any more.

The apache error log says: [error] child died with signal 11.
I´ve searched a lot, but neither space at functions.php and wp-config.php nor deactivating plugins and themes solve the problem.

Could WordPress MU Domain Mapping cause the problem? I´ve deleted it (at the database too), but the error stays. The server IP has changed, but I don´t know, if somewhere this IP is set.

I would be happy, if somebody has an idea.

Thanks!