I've installed the latest version (5.14 at this time) of Bitbucket Server, and I upgraded git to the latest version (2.19), and several repositories (annoyingly, not all) have stopped working. After rolling back git to 2.18 (or in another case, to 2.7), they seem to work again.
As an experiment, I created a new Bitbucket Server installation, and I started mirroring all repositories from the real server to the test server:
git clone --mirror http://source.example.net/scm/test/some_repo.git C:\TemporaryFolder
cd C:\TemporaryFolder
git push --mirror http://destination.example.net/scm/test/some_repo.git
My end result is that about 50% of the repositories get mirrored without problem, the other half crash, with the following error message:
fatal: the remote end hung up unexpectedly
fatal: the remote end hung up unexpectedly
error: RPC failed; HTTP 504 curl 22 The requested URL returned error: 504 Fiddler - Receive Failure
I've done this both on a Linux installation and a Windows installation. From the Windows installation and some procmon magic, it seems that, in the end, this commands fails (with exit status 128):
git receive-pack --stateless-rpc .
Of course the problem isn't really with mirroring; the problem manifested when we couldn't pull from the existing repositories (after upgrading to Bitbucket 5.14 and git 2.19)
EDIT: Disregard the line about fiddler, the reality is that there was no reply at all (because the process on the other side had crashed)