I rolled back to 2.0.4297 and no longer have that issue. This is the first time I've seen this particular problem.
Normally, as an admin, as soon as I click 'Connect to Server', I see "IP:Port / SteamID64 / Playername has auth level 2" in my server console. With uMod 2.0.4300, I do not see this message in the console and the connection immediately fails with the above error.