Problem connecting external Database in Joget onDemand

Hello,

i try to connect to a google cloud sql mysql database (version 1).
Access from all IP's is whitelisted, user account has global privilieges (tested with other app, works).

But when trying to use it in jdbc binder, i always get

Not able to establish connection.

Cannot create PoolableConnectionFactory (Could not create connection to database server.)

my JDBC connect line is
jdbc:mysql://XXX.XXX.XXX.XXX:3306/YYYYY 

interesting is: when i dont use the correct database name as YYYYY, but a wrong one,
i do get the message:

Not able to establish connection.

Cannot create PoolableConnectionFactory (Unknown database 'ZZZZZZ')

So it looks like it actually can connect...

Any hints? 

Thank you!
Cee Bee