messages like this in the mail logs:

warning: milter unix:/var/run/dkim-filter/dkim-filter.sock: bad protocol information: 6

typically indicate that someone followed very out-of-date instructions and/or
upgraded postfix or a milter without adjusting forced protocol usage in e.g. the above message was the result of using 

milter_protocol = 6

in when the milter did not in fact speak protocol 6. Removing or
commenting out this line fixed the problem.