GitLab 9.5.5 - Import project from bitbucket not working

Hello.

I'm having a problem when trying to import projects from bitbucket.

I've enabled BitBucket OAuth following this guide
https://docs.gitlab.com/ee/integration/bitbucket.html

And I've configured the required enviroment variables
- OAUTH_BITBUCKET_API_KEY=xxxxxxx
- OAUTH_BITBUCKET_APP_SECRET=yyyyyy

The problem is that when I navigate to New Project -> Import From BitBucket -> Import, a progress spin appears inside the import button but the project is never imported.

Start with the Bitbucket OAuth guide and the configured OAUTH_BITBUCKET_API_KEY and OAUTH_BITBUCKET_APP_SECRET values, then reproduce the failure through New Project → Import From BitBucket → Import. Trace the import request and determine why it remains in the loading state; done means a Bitbucket project imports successfully or the failure is reported clearly.