short update on this one:
I tried to reconstruct the errors with a clean "sandbox" installation of my version (4.3.5 JBOss 4.2 bundle) - to check if some of my changes messed the ugrade up:
The result is as before: 4.3.5 to 4.4.2 works fine - 4.4.2 to 5.0.1 fails.
The upgrade routine is run - but the jboss throws the mentioned errors.
BTW: The browser displays the following then:
An unexpected system error occurred.
javax.servlet.ServletException: java.lang.NoClassDefFoundError: com/liferay/portal/kernel/security/permission/PermissionChecker
Next thing I'll try is: 4.3.5 -> 4.4.9 -> 5.0.1.
If that fails i try to go JBoss 4.3.5. -> Tomcat 4.3.5 -> 4.4.2. -> 5.0.1.

One more question:
How is one supposed to deploy the war file to the JBoss? - copy the contents to the ROOT.war folder? or
- copy the war file into the deploy folder (there will be no ROOT.war folder created then..)
As always thanks in advance - and I'm gonna keep you updated...
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