First, install the new patch for CC 2015 and see if that helps. If not, try re-installing MSVCR120.dll (available on the Microsoft site).
And fully patched versions CC 2014 CAN open CC2015 files if they have internet access to use the cloud service for conversion.
Thanks, Peter. I installed the CC 2015 patch (via the CC desktop app), but it had no effect. (Or did you mean something else other than the update that just came through?) I am hunting for the correct version of MSVCR120.dll on the Microsoft site... the ones that seem to come up first are old versions. I tried a couple of them, no dice.
However, you really helped me by pointing out that CC 2014 can open a CC 2015 file. I'm back up and running in CC 2014 and I think I'm going to stay with it until I hear these bugs are worked out. Very glad to be productive again!
That was the patch I meant.
I suspect, too, that the .dll may be a red herring. If CC 2014 is working it's not really likely the .dll is damaged as I'm pretty sure it's using the same file as 2015. I see copies of it here in Windows\System 32\, SysWOW64\, and in \Program files (x86)\common files\Adobe\OOBE\PDApp. You could try copying one of the first two to the last location.
Agreed, I don't think it "really" is the dll having gone bad. It's just where the bug in InDesign CC 2015 causes a hangup. I am sure they will figure it out. If not, I'm stuck with CC 2014, which I don't mind.