A hero appears!
I think I found the cause.
Triggers with a 0 second timeout periodic event will cause a freeze. Before they would only cause a freeze if left on. Some GUI trigger systems, for example some “stun system” I ran into, used such triggers to alter execution order. They would have the trigger turned off and turn it on for 1 iteration when required after which the trigger would turn itself off.
The bug now is that Warcraft III will freeze after ~15 minutes with any such event. Does not matter if the trigger is turned off, never turned on, never referenced or anything. As long as a single trigger anywhere in the map has a periodic time event with 0.00 period the map will freeze after around 15 minutes of play.
The only case I know of is with 0 second timeout periodic timed events. However it is possible other timeout will also generate this freeze. Avoid using a periodic timeout below 0.02 as that matches approximately with Warcraft III internal frames.