@Alphonse - I can't seem to replicate this error on your shop. My first guess when I see that error message would be that you (or a staff member) may have authenticated the app with a Google account that doesn't have access to the Merchant Center that you were previously publishing to.
There's no way to downgrade to a different version of the app. Google shut down the old version of the API at the end of February, so the V1 API calls don't work anymore. If you're still experiencing problems, we'll try to sort them out as quickly as possible.