It's been over a year since I dug into this problem but I believe that the ports issue is inherent to all R parallel code, at least on Linux. Peeking at the futures code multisession is build on the "clustering" type of parallelism which means that it is vulnerable to this issue.

I haven't encountered the problem on Mac but that's because my laptop is generally unable to create enough parallel jobs to saturate the number of available ports.