I have never heard of WoltLab.
I did some researching and found that it's called WoltLab "Burning Board" - their forum solution. They seem to have a number of other online software as well, especially CMS (which after all is pretty much the core of a forum anyways).
I noticed some negative reviews about their documentation with people saying it was hard to convert their forum to WoltLab, and some feedback about plugin/theme availability, but most of the feedback is positive. The biggest thing I think you will struggle with is being able to quickly push out new features because it seems that WoltLab does have some missing features that come included in other forum software.
Overall though, I really like how your forum looks. It's clean, modern, and it looks no different than what I would expect from a more expensive & well known forum software.
Good luck with everything 🙂 I am definitely going to look into WoltLab a bit more.
You also gave me an idea for a new article - Lesser Known Forum Software 🙂 !