Invalid device function usually means that the code you are trying to run was not compiled for the GPU architecture that you are trying to run it on.
This shouldn’t be an issue with DIGITS itself (as that is basically a graphical front end for a DL framework) but is presumably arising either out of Caffe (or whatever framework you are using in DIGITS) or the cuDNN library itself.
You could try updating cuDNN to the newest version. This would likely require you to rebuild (at least re-link) Caffe.
If you installed DIGITS and Caffe and cuDNN using a package manager install method, then this may be a substantial amount of work, at least compared to what you have done so far.
The problem mentioned in the first post in this thread is actually a problem in the nvidia caffe build (0.14.2) that is pulled with the package manager install of DIGITS 3 (the cuDNN library is OK). The problem should be fixed in some upcoming 0.15 NVcaffe packages.