I have configured Drone in Local VM and created OAuth in bitbucket server and integrated with that and current version of Bitbucket is Atlassian Bitbucket v4.3.2. Both containers are up and running fine. While trying to access Drone from browser, it is authenticating with bitbuket. Once it is done it is unable to redirecting to Drone UI. Please find the below snippet for further reference.
I would expect Bitbucket to redirect to Drone (that is how oauth is supposed to work). I recommend contacting Bitbucket / Atlassian support to understand why the redirect is not taking place. Their support team should be able to advise further.