CORS issues breaking Stitch (AI Ultra Account, EU)

AI Ultra User, EU Region - For the last week I am unable to work on new projects - any of my assets are not accessible and any of the results from Stitch are not accessible (I briefly see the design work being done and then everything becomes permanently ‘working/loading’ animation). The problems occur with both entry sites - the KS console logs refer to a CORS conflict between the app-companion-430619 instance and the contribution_dot_usercontent_dot_google_dot_com domain with the user error:
Access to fetch at '``https://contribution.usercontent.google.com/download?c=[omitted]``' from origin '``https://app-companion-430619.appspot.com``' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource
and resulting
"Failed to load resource: net::ERR_FAILED"
for contribution_dot_usercontent_dot_google_dot_com errors (sorry for weird way of writing the URLs, but as new forum user I am limited to 2 URLS per post).

EDIT: It does work with a non-AI account (only with basic models) - but I would like to use my Ultra (history and access to more models, even though the prompt of my Ultra also just offers 3.5 and 3.8).