This behavior occurs when installers try to use the Boxcryptor root for temp files.
We already have an exclusion heuristic to suppress this message for certain programs, but as you noticed, this heuristic has to be updated once in a while, obviously the installer ofVisual Studio 2015 is not detected properly.
If you want to contribute to this, please open Boxcryptor in Debug mode (https://support.boxcryptor.com/display/BCWINEN/How+to+create+a+Debug+log) and start the visual studio installation. You can cancel it as soon as the error message occurs. Then, send the log file to firstname.lastname@example.org referring to this forum thread and you will see your contribution in one of our next releases. Thank you in advance!