GCP / Google Workspace Add-On JavaScript Origin

Question

I have a Google Apps Script Add-On that will not authenticate any more. It throws the following error:

Not a valid origin for the client: https://n-kn26x-script.googleusercontent.com has not been whitelisted for client ID 1061.apps.googleusercontent.com. Please go to https://console.developers.google.com/ and whitelist this origin for your project's client ID.

If I check the window.location.orgin in the console, I get exactly that *.googleusercontent.com url.

Now whitelisting this url no longer works (it used to), it throws the following error:

The request failed because one of the field of the resource is invalid. Tracking number: c50***

The function that throws the error is the following and is run from the sidebar a Google Sheet:

function initClient()  {
        console.log('INIT');
        return new Promise(resolve =>{
        gapi.client.init({
            apiKey: API_KEY,
            discoveryDocs: [],
            clientId: CLIENT_ID,
            scope: "https://www.googleapis.com/auth/spreadsheets"
        }).then(function ()  {
            // Listen for sign-in state changes.
            gapi.auth2.getAuthInstance().isSignedIn.listen(updateSigninStatus);
            // Handle the initial sign-in state.
            updateSigninStatus(gapi.auth2.getAuthInstance().isSignedIn.get());
                resolve(isAuthenticated);
            }).catch(
            e => {
                makeModal('OAUTH2', $.t('gapiLoadPb'), false);
                console.log('OAuth2 GAPI Error: ' + e.details);
                google.script.run.consoleOauth2Pb(e);
            });
        })
    }

I've searched around a lot, most of the tips are around changing the project, creating new credetials, etc. I've tried all that.

I would really appreciate any help with this!