Using the new APMSQL Server feature in 10.5 the MOM log shows:
AGCSmartstoreControllerImpl: Maximum number of connections reached: 5
In the EM/MOM IntroscopeEnterpriseManager.properties file is the new property "introscope.enterprisemanager.restapi.sql.connectionLimit" which defaults to a value of 5 i.e.
# If value of this property is set to positive number then
# APM Data SQL rest api will be limited to number of paralel connections.
# Otherwise transfer is unlimited.
# Default value: 5
Uncomment the property & increase it, or set to 0 to allow unlimited connections.
Where there is a cluster, the property needs to be updated on the MOM and all collectors