I ran across this issue while trying to process an offline certificate request on a OCS 2007 R2 Edge server. The certificate was issued from Comodo. When we tried to process the pending request using the OCS Certificate Wizard, we would get the following error:
"The input file is not a valid PKCS#7 file."
The solution, in this case, is to make sure that you import the Trusted Root and Intermediate Certification Authorities first, before trying to process the certificate request. Once we did this, we were then able to get the certificate imported without any issues.