Hey @matthijswelle  Have you taken a look at these articles?  You might find the answer here.
Why am I overbooked?
Overbookings — how to find and solve them?
Let me know if you find a resolution (or just mark this as best answer). If you’re still having problems, just shout. 

One feature I really like about Mews is when you overbook a room type, it will automatically hold a room in the next category up, so that you have a space to put someone if the overbooking doesn’t get resolved. You just need to go into your Stay Settings and choose “Diffusive” under the Availability Calculation Strategy if you want to implement that. 

Is Matt effin with us? or just starting the convo? ;)