We have been using a keyword in our slogan. What do we do against someone registering this as a trademark?

You probably need to consult with a lawyer in France, Belgium, Netherlands or Luxembourg as soon as possible. I think many lawyers in France or Benelux can cover all of those countries or have associates in those countries to help them do so.
It is my understanding that registrations of marks in those jurisdictions confer rights as much as, or more than, the actual use of the mark so that registrant of the mark may have acquired legitimate trademark rights in the keyword. It is also my understanding that Benelux registers applications very quickly, perhaps with limited examinations, probably in only 30 days. You might still have the ability to oppose the application or cancel the registration but you need to check with an attorney promptly. You might also have the ability to register your mark in the European Community, which would cover all of those countries, and if the Benelux registrant opposes that application, you will battle them there. If not, you may acquire sufficient rights to protect your mark. But you need to check with counsel there about this since I am a U.S. attorney and not licensed to advise about those countries.
The designation "combined" probably refers to the fact that the mark is both a word mark and contains a design element or logo.
* Please note the disclaimer in my bio.