We had cause to change the domain server for one of the windows domain.
Once SD+ had started it's daily scan, we noticed that it hung, and we could not gain access. if we re-booted, we could gain access for a short time, and did have time to change the domain controller it needed to use for the 2nd windows domain.
We continued to have issue for some time, before it finally came right.
The issue seems to be that if a domain controller is not found when it starts it's scan, it does not fail gracefully and carry on. It appears to lock out all access.
Not a common event to change a domain controller, but there might be an impact if the domain controller goes down, and a regular scan then kicks in.