I fixed the problem.
For all who have the same problem, here is what i did:
1. I fixed the bug in ispconfig and dkim signing, like Nicram...
Oh my fault.
Now the link works but the problem still remains.
amavisd showkeys is ok
amavisd testkeys => invalid (public key: not available)
Yes it looks similar, the data is in /etc/60-dkim but when I try to set the link
an error occurs:
ln: the symbolic link "/etc/amavisd/60-dkim"...
Thanks for your fast reply.
I added this code "dkim_key(mydomain.com', 'default', '/var/lib/amavis/dkim/mydomain.com.private');" to amavisd.conf...
I'm having some problems trying to get dkim to work.
After I updated ISPConfig 3.1.13 (CentOS 7), I set up dkim for one domain.
Separate names with a comma.