The Git LFS 2.0 client has been released and supports the new Git LFS File Locking API. Bitbucket Server currently (as at version 4.14) does not support this API.
The Git LFS client, upon attempting to lock a file will report the server does not implement the locking API:
Where an SSH remote is used the error message indicating "lock failed" will be different, it will report as an "Authorization error" as below:
The reason for this is, when authenticating via SSH a JWT authentication token is returned, for use on the Git LFS HTTP based APIs. While the token is valid for accessing the Batch API (for example to upload a file) it is not valid for the (non existent) /info/lfs/locks API.