What version are you running?
2.5.2
What's the URL of the page containing the problem?
https://our.reviewboard.de/r/12345/
What steps will reproduce the problem?

Configure the repository to have a bug tracker URL like custom:some/path/test?id=%s
Create a review with an associated bug
Try to click the link with the bug ID


What is the expected output? What do you see instead?
I would like my browser to open the link.

Instead I get a site showing "Bad Request (400)"
What operating system are you using? What browser?
Debian Linux 8. Browser is Chrome.
Please provide any additional information below.
The log says 
Unsafe redirect to URL with protocol 'custom'

Traceback (most recent call last):

  File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 112, in get_response

    response = wrapped_callback(request, callback_args, *callback_kwargs)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/reviews/views.py", line 1636, in bug_url

    return HttpResponseRedirect(review_request.repository.bug_tracker % bug_id)

  File "/usr/local/lib/python2.7/dist-packages/django/http/response.py", line 459, in init

    raise DisallowedRedirect("Unsafe redirect to URL with protocol '%s'" % parsed.scheme)

DisallowedRedirect: Unsafe redirect to URL with protocol 'custom'
I even found a possible solution on Stackoverflow:

http://stackoverflow.com/questions/14558876/django-1-4-redirect-to-non-http-urls