Verily, there was. Paranoid as ever, in System Preferemces/Security/Firewall I had reset from Set access for specific services and applications to Allow only essential services. Oddly, this does not seem to include TCP/IP networking. Restoration of the status quo ante solved the problem.

Oh! What a tangled web ...
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(60x) 13DT + 3PB + PTPro; (G3) 7DT/MT; (G4) 3T + PB. System 8.1 to OS 10.5.8