What mappings do you have configured (on the Settings page in the menu)?

How is your VSCode workspace defined? Should be using isfs with something like:
isfs://yourserver:NAMESPACE/csp/testdb?csp

Jani, thanks for asking - December 16 is the plan.

I definitely haven't seen issues like this with Apache, and the trailing "///" in ^XVMC("base") is definitely suspicious. It's very possible that the "clever" approach of having a .csp page serve things under further URL segments doesn't work nicely on nginx.