Hi @TomvanLoon!
Have your issues been resolved already? We didn’t observe any major outage on our side. So the most probable scenarios are either that there were some works on the Looker Studio side again, or your credentials actually got invalid (for example when your license for Piwik PRO expires, the API credentials are no longer valid).
Also, since this post is extremely popular, I would like to mention that we have a very fresh update of this connector with major performance improvements and much more. I hope you will try it out!
If your issues continue, @anthonybartczak will help you out.