I too am experiencing the same error. Here is my work around. Open the stencil file (vss or vssx) by a double click of the file via Explorer. The stencil will open in another session of Visio. Open your working drawing in another session of Visio. Now you can drag objects from the stencil only screen to the working drawing. You can also drag them into your "Favorites" of the working drawing and they will save.
Something of a pain to open two sessions, but it works. Recommend using dual monitors if you have the option.