Multiboxers have a problem where (occasionally) simultaneously zoning multiple characters can cause a freeze that lasts until they forcequit one of their characters. Some people experience this much more often than others and it's not clear why.
I just pushed Sandbox 1.0.1.9, which hopefully will fix that problem. It should stagger your characters (by 100ms) so that only one can initiate zoning at a time.
It's difficult to tell whether this problem is solved because it happens so rarely in the first place and I only dualbox. Please let me know if you still experience the multi-box zoning freeze.