I upgraded by SQL Prompt 18.104.22.168 to 22.214.171.1245.
SSMS started correctly. But, when I first tried to access a database (and, I assume, when SQL Prompt 5 first began executing), SSMS went into an internal wait state. A task bar notification from SSMS appeared, stating SSMS " ... is waiting for an internal operation to complete. ..."
Rebooting did NOT solve the problem. SSMS again went into the internal wait state.
However, REPAIRing the installation from the Control Panel's Add Remove Programs applet did work.