Mail app cannot connect to iCloud account
I have an existing iCloud account for which I use a gmail address. I recently created a new @icloud.com email address.
I have several iOS devices and two Macs. All the iOS and one of the Macs can connect to the icloud.com mail account; the other can't.
Both Macs are on the same version - Mojave 10.14.6 (18G3020). On the problem Mac all the other iCloud services are working correctly; only Mail has a problem. The Mail app is configured the same on both.
Any ideas for troubleshooting?
Mac mini, macOS 10.14