This is still not fixed. Seems it's not possible to use Testflight again after resetting the device. I've tried everything. Just getting the invalid invite message.

Something is just messed up. I previously accepted an invite sent to my work email on a device that was logged in to a shared develper account. After logging in to the device with my own work email, I could no longer use Testflight. I seems somehow my work email is linked to the shared work email, and It's just not possible to fix. Then I sent an invite to my private email, and accepted it in Testflight with my work email. Now it actually works, but I really wish they could improve this.