With the addition of the Dev Panel, it seems that Bitbucket doesn't send anymore responses to the Source Tab if a Fisheye integration is still present. Instead it leaves an error message within the Source Tab above the Fisheye information as such:

This list may be incomplete, as errors occurred whilst retrieving source from linked applications:
Request to http://<stash-url>:<port> failed: Failed to parse the response from Stash: null

However, the Development Panel works fine as suspected.

Steps to test:

1. Created AppLink between JIRA 6.4.5 and Bitbucket Server 4.0.3.
2. I had an AppLink already created to a FishEye instance - so I had the Source and Reviews tabs showing up in a JIRA issue.
3. Committed something to my Bitbucket repo, referencing a JIRA issue created for testing.

Results
Development Panel works fine, but the Source tab shows a "null" result when trying to get information from Bitbucket.

Additional Info:
Fisheye was only mentioned because that's the only way we found the Source tab to be present.

Resolution
Support for Bitbucket Server was added in JIRA FishEye Plugin 6.3.10. JIRA 6.3.x and 6.4.x users can get the upgrade either:

1. Downloading it from Marketplace.
2. Or using the 'upgrade' functionality on the 'Manage add-ons' page.