We have an active svnmirror sync in use. On the git site we have some binaries like dummy.exe and after syncing with svnmirror in to our svn repo there we have the same files but they have length zero and are corrupted.
this sounds very similar to an old bug we resolved in version 3.3.3 – it was an error that led to that binary file more than 50 MB in size were translated to zero-lentgh files in SVN. If you are using a version prior 3.3.3, then it may be it and the solution is to upgrade SubGit to the latest version. If you already have the latest SubGit, then it appears to be an unknown issue that needs investigation. For the diagnostics, please provide us with all SubGit logs (they reside in subgit/logs directory in the repository).
it looks I misled you here, my apologies for that. I thought you had SubGit command-line tool and I was talking about SubGit version, not about SVN Mirror add-on for Bitbucket Server that you have. The version 3.3.12 of the add-on is based on even earlier SubGit version and it does have this bug. So to resolve the issue please upgrade the add-on to the latest 3.4.10 version as it suggests.