NAS connected via ethernet: "There was a problem connecting to the server "---.---.---.---"

I get this error whenever I connect to my shares on my Synology NAS, which is connected to my iMac via ethernet.

The frustration is that there ISN'T a problem. At all. The iMac then goes ahead and connects.

If my iMac goes to sleep, and loses connection, I need to reconnect using 'Go'-->'Connect to server' in finder, I get the same error. Again, on clicking 'OK' it connects anyway.

It's an afp:// address.

Why am I having this issue? I'm also using a PC, and that connects and stays connected, even from sleep, via Wi-Fi, but my iMac complains of a problem whenever connecting to shares on my NAS on either Wi-Fi or ethernet.

I have recently done a clean install of Catalina, and this issue didn't go away.