Git client: Cannot push to private repo not owned by me

[Frank Ralf]

Hi,
I am working with a private GitHub repo which I do not own but have write access. I can push to this repo with Sourcetree and from the command line without problems but with the Oxygen Git client I get the following error message:

"You have no rights to push in this repository"

I have reset all credentials and tried both personal access token as well as basic authentication but to no avail. I have read the following posts but could not find a solution.

• Git Plugin fails to push, doesn't ask for credentials
• Git staging does not push files to BitBucket repository

Is there anything else I can try?

Best regards,
Frank

EDIT
Cloning the repo with the Oxygen Git client does not work either.