VPN problems with PEM file import

Hello,

on a Cisco 831 with IOS 12.3(8)T we try to import a PEM file for the CA.

I tried:

crypto pki trustpoint blabla enrollment terminal pem exit

crypto ca authenticate blabla

After I paste the PEM file into the terminal I get an error message:

Error in saving certificate: status = FAIL

Are there limitations for the format of the PEM file? Can somebody please point me to an example config where CA authentication is done with PEM files?

Sincerely, Stefan Lieser