Use the leak checker, file->leak check->load leak file(.lin). It draws a yellow line in 3d view from one of your entities to the leak, it's a little hard to follow but worth it. And yes that error is related to the leak, vis is having a hard time doing its thing, vis + leaks don't play well together.
Also I think something like a ladder or a cave behind the flag that goes to a short, icy low path would really spice this map up, possibly with a way in and out of the low path in mid somewhere
. Or possibly have 2 caves in mid, one goes to red low and the other goes to blue low but the lows arent really connected, they just both empty out into mid. That would be kinda cool.