As a precaution, I immediately replaced the transformer: LED Transformer 12V DC 24W max. Transformer driver. The old transformer needs at least 20 W power loss. However, I could not notice any problems with the old transformer with a short test. So I do not know whether the replacement was necessary.
The lamps operate on 12 volts DC or AC power. In DC, it does not even matter Wierum they are used. Anyway, I could not notice any problems.