Thanks for your reply.
This thing is that I have just reinstalled everything. It's been a while since I played a computer game because we have a newborn baby at home that kept me busy. So I installed Colonization fresh (via Steam), installed the latest WTP and started a new game. All worked well. So I assume all is installed correctly and I also assume there is no bug because the game worked well after the installation.
I am a bit puzzled because the error points towards something that is not in the original game but only in the WTP mod ("faireweathertweakex") but it comes up even before loading the WTP mod. I am not a programmer or modder but to me it looks like it is a cached information, maybe the core game remembers a setting and then after a reboot it is not there. In that case the error would probably reappear after the first game, even if I reinstall everything again now. The solution might be to clear that cache in the core game so I can launch or get to start the WTP mod directly from Steam without starting the core game at all.