CMIP6 fetch causes CORS Header errors sometimes

[Clichee]

Issue

Fetching data using the cmip6 server causes it to get cors errors and also sometimes errors on the backend-server (badzip, fileNotFound, ...?).
Adding a firefox add-on to automatically add cors response headers, seems to resolve the issue mostly (sometimes httpError 500).
CORS Header may be set incorrectly on the backend side.

Steps to reproduce

1. Run webapp and backend server
2. Choose CMIP6 as your source
3. Select data for random dates
4. Observe error in browser console at some requests