Problems with file sharing

[lomontman]

My wife and I each have iMacs and I have a MacBook Pro. I often use either my iMac or MBP to access files on my wife's iMac. I've been doing this for many, many years without a problem. I just upgraded my wife's iMac to Mojave. I'm still running High Sierra on my iMac and Sierra on the MBP. After installing Mojave I immediately started having problems with file sharing. The Public folder on my wife's iMac is not accessible. File Sharing is turned on in System Preferences and the Public folder is listed as a Shared folder. If I choose my wife's iMac under the Shared heading in a Finder window sidebar I’m told that “Connection Failed” but the Disconnect button is available at the top right portion of the window. A drive that is connected to a USB port for backing up is available though. It gets weirder though. This behavior is not consistent. As I write this I can connect to my wife's computer from my iMac but not my MBP. I also cannot connect to my iMac from my MBP and vice-versa. I can connect to my MBP as a registered user, but cannot connect from the MBP to my iMac as a registered user. So perhaps the problem is not Mojave on my wife's iMac. Perhaps there is a more pervasive network problem. But why has it shown up only after upgrading my wife's iMac to Mojave? And what could this network issue possibly be that has never been a problem for many, many years?