Hello Przemek,
Welcome to the Testiny Forum.
In the Redmine integration this can happen if
- the Redmine API returns an empty list of projects with that API key
- the API key is for a Redmine admin which we do not allow but the error message might be misleading
We recommend creating a non-admin user and assigning permissions to access the projects required for the integration only.
If that does not work for you, please let us know.
Regards,
Michael