ai, the exact commands ? That was years ago, no possibility to retrieve those. Anyway, I followed this guide quite literally:
http://www.dd-wrt.com/wiki/index.php/VPN_%28the_easy_way%29_v24%2B#Creating_Certificates_using_Ubuntu_Linux
Update, I've checked a bit more careful the cert.pem file and it was not complete, it was missing a few lines at the bottom. In dd-wrt, the keys/certificates needed to be entered in a web frontend, maybe something got lost there. Anyway I re-copied the file from the original location (I still had them, from back in 2009) and now at least the server starts fine. My fault in bad checking.
Now the next problem, the connecting of the client of course. After providing an accessible NTP server -my company filters out the outbound (internet directed) NTP requests- I got a proper date and the certificates were valid again. The client connects fine, receives an IP address from the defined range and I can ping myself and the VPN "gateway". I can't see my servers because I'm not home, I'm just doing this in some spare time in the office.
Mission accomplished, it all works in my simulated environment in the office. Hopefully it works this evening at home as well.
Thanks for hinting towards the cert file quality.