On startup slicer looks in the settings and if it finds a TemporaryPath it tries to use it without checking validity .
On mac, the temp dir can change from time to time and an old path can become invalid, for example when upgrading the OS. Also it is machine-specific, so a user that copies their home directory from one machine to another can end up with an invalid TemporaryPath.
The problem shows up when Slicer uses the temp path for something like saving an MRB.
Here's an email exchange between me and Ron that discusses the problem and the workaround.
Deleting Slicer.ini did the trick.
Yes, it's probably because you still have the settings file from the older installation but the paths have changed.
You probably need to use a terminal to do it since it's invisible in the Finder.
If that turns out to be the issue we can file a bug report. This is the code that needs to be fixed.
It gets the temp path from the user settings but doesn't check that it is usable.
I just got my new mbp 2016 running the latest version of Sierra. When trying to save an mrb, I get the error message below.