blank screen for several pages

[p-schneider]

After I log in (as admin) the page "Start" (main page / frame-> https://myserver:8443/admin/home/admin) is blank. I can still see and use the menu on the left and the bar at the top of the page.
The file /var/log/sw-cp-server/error_log shows:

2013-01-26 17:29:24: (mod_fastcgi.c.2746) FastCGI-stderr: PHP Fatal error:  require() [<a href='function.require'>function.require</a>]: Cannot redeclare class adminpassword in /opt/psa/admin/plib/Os.php on line 16

I get the same error (blank screen, same log message in /var/log/sw-cp-server/error_log) when trying to check for updates via the webinterface (https://myserver:8443/admin/update/product/)


When I edit (open) a domain in the control panel (https://myserver:8443/admin/subscription/login/id/38/ forwarding to https://myserver:8443/smb/admin-home) the page seems to be fine, but it triggers:

2013-01-26 17:43:30: (mod_fastcgi.c.2746) FastCGI-stderr: PHP Fatal error:  Cannot redeclare class APSCategoryConfiguration in /opt/psa/admin/plib/SiteApps/APSCategoryConfiguration.php on line 6

When I try to Manage the DNS settings for a domain (example URL: https://myserver:8443/plesk/client@13/domain@38/dns/) I get a blank page, too. This time the file /var/log/sw-cp-server/error_log shows:

2013-01-26 17:36:15: (mod_fastcgi.c.2746) FastCGI-stderr: PHP Fatal error:  require() [<a href='function.require'>function.require</a>]: Cannot redeclare class message in /opt/psa/admin/plib/common_func.php3 on line 1991

Sometimes I get errors like:

2013-01-26 17:35:37: (connections.c.1762) SSL (error): 5 -1 32 Broken pipe

but I guess this is normal

There are no (new) errors in /usr/local/psa/admin/logs/panel.log

I already used bootstrapper.sh repair and did upgrate to the latest version using the command line based installer (automatic updates are enabled, too).
I am using Ubuntu 12.04 x86_64 - The server was installed end of June last year based on a fresh Ubuntu 12.04 installation. (I installed Plesk using parallels_installer_v3.12.0_build120601.16_os_Ubuntu_12.04_x86_64)