Hello, I’m trying to setup Drone CI using BitBucket, but it seems there’s some problem. I always keep getting error Invalid redirect_uri. I’ve tried two different accounts and gave all the available permissions, but it was the same. Then I tried GitHub with the same info and it worked.
I can confirm that I am using Drone + Bitbucket and I do not have any issues with authentication or redirects. If you are having issues with Bitbucket, and not GitHub, perhaps your Bitbucket oauth application configuration  is incorrect. Based on the error message it sounds like a problem with your Callback URL configuration. This is the only explanation I can think of.
Seems I have found the issue: as I was deploying Drone using GCloud VM instance using their container configuration, where you provide image,arguments, env, volumes. And it seems the arguments were processed, because the host address would get and additional http:// appended so it looked like http://http://<ip>. Once I changed the host to - DRONE_HOST=<ip> without the protocol it worked.