I have a QNAP NAS so I'm unfamiliar with the software from Synology. QNAP has a small program called Q-Finder that seeks out the currently assigned address of the NAS. When that is determined, I put that address in the NFS "Add Client" or "Modify Client" config and then directly access it.
This address can also be determined, though more awkwardly, from logging into the router interface.
I finally realized that once that locally assigned IP address was determined, I could assign that through the router as a static IP so that the NAS will always have that particular address.
In your situation, it may be that the NAS address has changed so the Med is waiting for the new address to be added through NFS. In that situation, if you try to access a previously added NFS client, the Med will fail because the NAS is no longer at that address.
For housekeeping purposes, any previous clients listed in the Med's NFS interface will be useless and can be deleted. If you assign a static IP to the NAS and add that to the Med, and then also delete the previous client IP addresses, from that point forward you should be able to breeze right through the process and avoid login errors because the one remaining client will always be the correct address for the NAS.
I hope this helps, pdlb.