Hello @AlexSz,
I see you are using an Enterprise Organization and also tagged your post with self-hosting
You mention not having special characters in the username, but still getting the error
user xxxx has already been invited to this organization yyyy. Accept the invite in order to log in with SSO
If you are using SSO, do you know if perhaps your self-hosted instance is also utilizing the Bitwarden key connector with login with SSO?
If so, you may wish to login to the admin interface of your Bitwarden, (or have the appropriate system administrator for your self-hosted instance for your company’s IT department login) by going to whatever your Bitwarden self-hosted domain followed by /admin
i.e bitwarden.company.com/admin to see what the User’s status shows.
Here is a similar posting with a user that experienced an issue with key connector for login with SSO, when trying to re-add a previously removed user from an Organization.
Baring that, I would recommend to reach out to Bitwarden’s support directly and to get in contact with their support team for further assistance.
Hope this helps 