When I open Blocked Sites in System Manager the software stalls and I'm unable to browse the blocked sites list. I have to terminate System Manager in Windows. using 12.9 update 1 on a Windows 10 Professional version.
This is a bug -- it is FBX-24474
We're hoping to have it fixed in the next release (12.9.2.)
You can use the WebUI to manage the blocked sites list in the meantime.
This issue tends to appear for customers that have very large blocked sites.
WatchGuard Customer Support