I encountered the same error but my workaround was different: I upgraded first to 6.0 U1b then to U3f. Directly from 5.5U3 to 6.0 U3f results in the prerequisites check fail with the certificate validation problem. However I suspect this to be a false positive and an error in the prerequisites check as if it works with the double step upgrade it cannot be a certificate error. BTW we use no external CA and had self signed VMWare installer certs.
An extra hint: if you encounter a VCSServiceManager installer error you have to rename or delete the wrapper.conf file as explained here: