I was having that "the original item can't be found" error for years when clicking the finder entry for a Synology NAS share. I finally decided to do something about it. After much research I decided to switch over from connection type AFP to SMB (Samba). I have read that AFP as a connection type is being phased out but I don't know that for sure. Anyway it was always dead slow, for me, and kept erroring-out just like yours is doing.
If you decide to go the SMB route there are some discreet things you have to do to get the Samba connection working well. This includes turning off "signing" (encryption of the link conversation) on both sides, because it makes things way too slow, and after all it's my internal LAN, with no untrusted devices connecting.
Unfortunately I did not document each step of the way as I bumbled through it but the highlights were:
First I made sure I had a good backup.
Using the NAS's DSM config, turned off AFP and made sure SMB was already enabled (it was), and SMB signing was turned off. To do this go to to DSM > Control Panel > File Services. Turned off samba signing using the Advanced Settings button in the SMB area, set Transport encryption mode to Disabled.
On the mac, used terminal to disable signing as a default for samba connections, command is:
printf "[default]\nsigning_required=no\n" | sudo tee /etc/nsmb.conf >/dev/null
Rebooted the mac a couple of times at various points, and also relaunched finder a couple of times.
On the mac, reselected the Time Machine backup drive (the NAS share), which looks just like the old one.
I have to say I'm really glad I did this, it's now very fast, responding almost like a local drive. Samba connections appear as type "CIFS" in DSM > Main Menu > Resource Monitor > Connected Users. Time Machine backups are all still there and work fine on both save & restore.
The "original item can't be found" message has not reappeared since I did this, about three weeks ago, on Sierra. Since then I've moved to High Sierra with no issues. Hardware is MBP mid-2012
