Also having this issue, not a new problem. Surprised there are not more posts about this as it is entirely blocking usage of a critical gpt for us. Was working fine until we updated a custom action, now OAuth fails every time across devices and browsers. Reverting to the previous action code did not fix it, seemingly no way to fix from our end
EDIT: Deleting and re-creating the CustomGPT solved the issue, though this is not a reasonable long term solution