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.
joymyr
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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.