One Single User cannot connect to OSX File Sharing

OSX Mojave 10.14.6.latest used for All Macs involved.

I share from one 2019 Mini-Mac (64GB Ram spec.) to 12 iMacs.

Everyone can connect for for days, weeks even :-)

Then for no reason (that I can find) just One user will be dropped or cannot connect.

It is just One user.

Doesn't matter if I change that iMacs IP.

I clean out their KeyChains for the Server Mac connection on that iMac too.

I reboot the iMac in question... no change.

Nothing solves the issue.

Only restarting the Mini-Mac 'Server' that is doing the sharing will solve the issue.

In the old days of having a working supported full OSX Server App I could perhaps disconnect the user on the Mini-Mac 'Server'.

I'm stumped :-(

Thanks for reading ...