"Problem connecting to server" on one computer, not the other

Two computers; one connects fine, the other gives me the "Problem connecting to server" dialogue.

The server in question is the Time Machine volume on a Western Digital MyCloud NAS. One is supposed to connect as Guest. The server appears in the Finder sidebar under "Locations" on both computers; but when I click "Connect as..." on one, I get a connection dialogue, whereas on the other, I get the error dialogue.

I thought I might have a corrupt Keychain, so I deleted all password records relating to this server. No change.

For the life of me, I can't figure out what the difference is between the two computers. They are both Big Sur. Different usernames, but why should that matter, since I'm logging in as Guest? Any suggestions would be appreciated!