Reaper 6.11 freeze on my system Mac El Capitan is highly erratic but fine with Reaper 6.04. See attached screenshots. The 1st is a freeze of a duplicated track that shows the anomaly in 6.11. The 2nd the same song but frozen using 6.04.
I suspect it is system related as surely everybody would be noticing this. It's very strange.
I've noticed that if you have two tracks with the same name, or no name, and you try to freeze them you get an error message. I suspect, it try to create two files of the same name and runs into trouble because of that.