Submitted a new beta build yesterday and am seeing the same issue. The app wasn’t showing up in TestFlight, so I deleted my email from the ‘All’ list of testers and then added it back to the internal list. Clicking on the invite produces the error message ‘TestFlight is currently unavailable. Please try again later.’ This appears to be an issue with my phone, because others on the development team have been able to install with no issues.