Perform the following steps on the OMS server as a work around
1. <OMS_HOME>/bin/emctl set property -name use_pooled_target_connections -value false
2. Restart oms
<OMS_HOME>/bin/emctl stop oms -all
<OMS_HOME>/bin/emctl start oms
Note: EM 13c: Target DB Home Page Shows 'Failed To Connect To The Target Database, Regions That Pull Real-time Data' (Doc ID 2474441.1)
Performed as you mentioned still got the same error message.