Thunderbird is the client and yes, due to the fact that I have at least 20 users logged in the program default of 50 is the issue. Adding the "MaxConnections" option (LONG AGO) solved the issue, BUT it came back when the settings from the .INI files were converted to registry entries (Client ver. 20.7 I believe).
The needed entry (see my previous post) was migrated automatically by the upgrade and it is in the registry but obviously not working. Thunderbird by default uses 5 connections per user... as soon as I pass 10 users, the errors start. As I said, I dealt with this issue LONG ago and the .INI entry was working... until the upgrade.
Notice the LOG's second line contains "Processes: thunderbird.exe[50]" which I might indicate that the client default has reverted to 50 as I suspect.