The DK Key Path (or DKIM key path in case of DKIM) refers to the location of a file containing your private key. You need to generate a private/public key pair using OpenSSL as indicated in the documentation (the private key will be used in Axigen for signing, and the public key will be used in order to generate the required DNS record).
The error is likely related to the content of the file you specified as DK Key path / DKIM key path in the rule that signs your messages. Please note this file must contain your BASE64-encoded private key only - this should not be a SSL certificate.
I left just the private key in the file. I didn’t get any errors. The message was sent and was received in the spam folder and in the header of the message I found this:
dkim=none (message not signed)
However on the server I can see the following in the log:
:Start filter onProcessing event
:The message has been signed using DomainKeys
:Finished filtering mail object 245FEC with filter: onProcessing event
According to the log sequence the message was DomainKeys signed.
Please note however DomainKeys and DKIM, while similar, are actually not identical and produce different signatures.
If you want your message do be DKIM signed, you need to add to your rule the following actions:
DKIM key path
Also note, you can add both signatures (DomainKeys and DKIM) if you wish.