Sorry for the delay. Our QA team have been trying to reproduce the issue in our environment. And they managed to do this.
We have never come across such an issue before. The problem is in the number of AD domains managed by your Adaxes service (I believe you have ~20).
To fix the problem right now, you'll have to unregister a couple of AD domains and restart the service.
A fix for the issue will be available in the nearest service pack for Adaxes. If you urgently need a fix, we can create a custom build for you.
Sorry for the inconvenience.