That doesn't seem to be the issue, as I am able to create new customization specs. However, I was able to find out that after clicking next, the following error shows up in the log:
Caused by: java.lang.NullPointerException: null
This makes sense as the form is probably not passing any IP Pool information to the sever as it wasn't able to get this information itself.
Is there something new in vCenter/vSphere that requires additional configuration for the network?