During the authentication process for Threads after the user logs in and accepts the app's scopes the user is presented with a blank screen that only displays:
Login Error
The url contains the following details: params[error_code]=1349003&source=gdp_error
This is only happening for users who have a Thread account tied to an Instagram business account (professional account)
If the account is a business account and a tester for the app it does work.
Looks like Instagram fixed the issue. We aren't running into that issue anymore.
I have the same issue, also for an account with a professional account. However, when I change the account to personal and invite the connected Threads user again for testing, it still will not allow access: same error (error code 1349003)
Very frustrating!
Could you please add that as a comment rather than an answer? It sounds like what you tried didn't solve it.