My application was accepted and is now suddenly rejected, what happened?

This is probably because there is something that makes your application invalid. Maybe you are based in a country where the job is not available, or your native language is not of the variant we require.
Please note that there is a difference between Accepted and Approved. Accepted means that you seemingly fulfill all the criteria and have passed the first stage. It does not grant access to the project. The Approved status does. If you never switch to Approved, it’s probably because there are more applications than vacancies and your application was submitted later than others, or you don’t fulfil some of the criteria stated above.
If you’re still unsure, please contact your Project Manager or Recruiter for clarification.