Works for me on version 8.4.
Make sure your function returns a string starting with T# (because it's a Filter). Maybe test it on a dummy cubeview first, to verify it actually returns what you expect (the Workspace might have to be set to Shareable for that to work).
I am on 8.2.2 and I tried hardcoding the return value, still getting the same error.
If you've tested with a Cube View using the same syntax, and it returns the right filter there, you will have to talk to Support, as it might be a release-specific bug.
Below is the code, I added logs to the assembly, but nothing getting displayed in the error log. I feel like the call is not correct, hence it's not able to read the function. I tried even removing the parameters. Still no luck.
Below is the assembly function
