FMS12 are not compatible with FMP14v6. You need to upgrade to FMS14 or higher. I recommend you to upgrade to FMS15
Assuming this is due to SSL (which it probably is), there is a patch available for FMS 12 that allows the newer clients to attach.
However, if you apply this patch, it will mean clients that aren't patched (v12 or v13) will suffer the same experience. As Johan said, upgrading is a better path.