Thanks for the answer .
Same error resolved for me in Hana DB as well after setting the DEFAULT key on central instance.
We were facing this issue
==========
** ERROR => Connect to database failed, rc=1, rcSQL=-10709
========= after seperating the ci from DB server.
when I saw the R3trans log it was showing
=========
========
4 ETW000 [ dev trc,00000] -> checkEnvironment(con_hdl=0) 95 0.035289
4 ETW000 [ dev trc,00000] -> connect(con_info_p=(nil)) 62 0.035351
4 ETW000 [ dev trc,00000] Try to connect via secure store (DEFAULT) on connection 0 ... 130 0.035481
4 ETW000 [ dev trc,00000] no connection data found for key (DEFAULT) in HANA secure store 588 0.036069
4 ETW000 [ dev trc,00000] connect via secure store, rc = -10104 (Invalid value for KEY (DEFAULT))
4 ETW000 22 0.036091
4 ETW000 [ dev trc,00000] Try to connect via environment (localhost:7878) on connection 0 ... 21 0.036112
4 ETW000 [dbhdbsql.cpp,00000] *** ERROR => Connect to database failed, rc=1, rcSQL=-10709 1219 0.037331
4 ETW000 [ dev trc,00000] SQLCODE : -10709
============
So, issue resolved after setting the DEFAULT key on ci.
Regards
Prashasti