Hi all,
as a fast solution you can also run the following command. Of course this only extends the problem for one month. But I hope there will be a fix until then.
openssl ca -config /usr/share/openvpn/ovpn.cnf -gencrl -out /var/ipfire/ovpn/crls/cacrl.pem
After the above command the CRL was updated and renewed and OpenVPN connections work again. 
I just can recommend to do this to everybody on V186 especially on remote machines before the CRL expires.
Cheers and have a good day
P.S. also changed the Wiki page https://www.ipfire.org/docs/configuration/services/openvpn/config/upload_gen