This scenario was first noticed when the data_engine queue turned yellow and started building up.
No data was making into the database and the data_engine probe sent an error that it's queue was building up.
In the data_engine GUI, we see 0 msg/min (0 msg/sec) and the 'test connection' button fails.
We tested telnet from the primary server to the database server over port 1433.
This fails when the UIM service is running on the primary, but succeeds when the service is disabled.
The netstat command (netstat -ano) shows a SYN_SENT (but no ACK):
TCP 192.168.0.1:50994 192.168.0.1:1433 SYN_SENT 3288