Sharan,
Thanks for your hint with the SAP Note. Unfortunately it did not solve my problem. I've done excatly what was described in the note and in the end all parts of the script said "success". However, I still get the same error.
What I found out, that the script wants the sum in the directory /usr/sap/SID/SUM, the Installation of Revision 53 puts everything under /sapmnt/SID/SUM - but in the end, it shouldn't affect the sum. However I also created links between the different locations and it does still not work...