can you share the file you are trying to upload? It sounds like a formatting issue with either the structure of the zip file or one of the files inside.
I would say it depends on if the 10.5 version was compatible with your CUCM version. I typically upgrade PLM (if standalone) first followed by CER then CUCM, CUC etc.
If compatibility is fine then it would just depend on how long CER was running 1...
