things are working now. Fortunately, we had a massive VMware storage problem today, thus we were able to update the older Open-E to the same version as the other one (because lots of things didn't work anymore, so we could shut down the complete system cluster).
Immediately, the replication volumes have been displayed as expected.
=> It doesn't suffice that the DRBD versions are the same. The main DSS version must be identical!
(It is not clear to me how one could migrate to a newer version without downtime under that restriction. This is a pity. Open-E should present a solution for this case).