Today I was upgrading 10gR1 to 10gR2 (10.2.0.1) on Linux x86. The upgrade went almost fine (except that I had to install one package and change few kernel parameters) but while running DBUA to upgrade databases, it gave an error:

Could not get database version from the Oracle Server component. The CEP file rdbmsup.sql does not provide the version directive

and

Start of root element expected. Upgrade Configuration file
'C:\Oracle10g2\cfgtoollogs\dbua\test\upgrade5\upgrade.xml' is not a valid XML file.

I searched in the metalink and found that this all happens due to customized glogin.sql file which was there in my case also. And removing that customization made DBUA rock 🙂

You might want to check here, here and here.