Synology recently released DSM version 5.2-5565 and with that came some updates for SMB v3 (SMB 3). By default, DSM is set to use SMB 2, and when you change the option to use SMB 3, it seems to “reset” all of your connections.
What that means is that ANY open connections or mapped drives will not work right away; you may have to disconnect and re-connect them all. I actually had to remove my mapped drives and then reconnect them to get them to function properly. I think that may be remedied by a reboot, but I didn’t try that (since I don’t reboot my computer very often).