Azure DevOps: 401 - Uh-oh, you do not have access

Question

My apologies to ask this question again, I'm aware that it has been asked before, but the solutions to those question don't work for us.

Some context

When I invite users to either my organization or project they don't have access and get the error as mentioned in the title. The users that I've tried to invite have personal Azure DevOps and are able to access another Azure DevOps environment. One of the people I've invited was able to access the project, but we're not sure why he was the only one to do so. All users got an Azure DevOps account by linking their GitHub account.

Things I've tried so far for the other users:

• Solutions listed here Azure DevOps '401 - Uh-oh, you do not have access' Invited User and Invited user Azure Devops project but they are unable to access it
• Made the project public and tried again
• Waited for a couple of days and tried the link again
• Made a new organization and tried to invite the same users again
• Deleted cookies and Incognito mode
• Tried above solutions in multiple modern browsers

So far we've had no luck with these solutions for the remaining users which have been invited.

Help would be most welcome.

Thank you for your time!