This error is returned in case an attempt to connect to the HANA database via a secondary database connection did not finish within 30 seconds and timed out. The time-out implementation is by default set to 30 seconds in SAP Kernel versions < 742 PL22 for secondary database connections.
The following entries in the corresponding dialog workprocess trace confirm that you were facing exactly this issue:
Wed Sep 2 14:10:06 2015
Try to connect via secure store (SERVICE) on connection 1 ...
Wed Sep 2 14:10:36 2015
*** ERROR => Connect to database failed, rc=1, rcSQL=-10709
Connection 1 is a secondary database connection and the difference between the connection initiation start-time and timeout-time is exactly 30 seconds. It may happen from time to time that connection attempts via secondary database connections take "longer"
Set the following SAP instance profile parameter on every application server:
dbs/hdb/connect_property = CONNECTTIMEOUT=0
After a restart of the SAP application server the changes become effective and the connection attemts should no longer time out.