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"