The doors just don't open after closing them and waiting a few minutes.
But the problem is solved. When I replaced the radio by a new one, I
didn't use a connector with 12V directly from the battery.
This connector was build in by the dealer and it didn't look like it was
standard from the factory.
This connector had to be connected between the standard ISO connector
and the radio. The 12V power on the ISO connector is only suitable
for the standard Citroen Radio. Any other radio requires 12V directly
from the battery.

Now the major question is why? and what is the relation of this to
the central locking and the remote key?
The car is controlled by a computer, Citroen calls this a BSI unit.
The car is also equipped with multiplex wiring, one wire carries multiple
signals, just like a computer network. The 12V from the standard ISO
is coming from this BSI. When turing off the engine and closing the doors,
the radio is still using a little bit power, to keep the radio station
presets
and other settings in his memmory. When the BSI detects the use of power
when the engine is turned of and the doors closed, it puts itself into
sleepmode, to prevent that the battery gets down. This also results that the
doors can't be opened by pressing on the remote key.

The dealer solved this problem. Some guys on a Dutch Citroen Forum
told me about this.