That sounds like a general Drupal problem rather than a problem with the change to multi-site. Have you definitely copied across all of the modules and themes from your old install?
The only other thing I can suggest is to try to copy the site from its old single-site install to a new single-site install, and see whether the same issue appears. If so, then you can be sure it's a general Drupal problem rather than a multi-site specific problem.
Hi Dominik,
That sounds like a general Drupal problem rather than a problem with the change to multi-site. Have you definitely copied across all of the modules and themes from your old install?
The only other thing I can suggest is to try to copy the site from its old single-site install to a new single-site install, and see whether the same issue appears. If so, then you can be sure it's a general Drupal problem rather than a multi-site specific problem.
Good luck!