It looks like the Switch On Gameplay Tag Container node stopped working in 4.15. The Switch On Gameplay Tag works fine.
1. Create a new project
2. Add a Switch On Gameplay Tag Container Node and hook it up into the flow
3. Click Add Pin
4. Select node, go to Pin Containers and fill it in with a valid tag
5. Compile the blueprint, it will fail
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45883 in the post.