Different queries, in different flows, fail with exactly the same exception - seems like a general problem with the interaction with the database.
Yoshinori, can you please attach server.log as well? maybe it will hint the cause for this.
Hi,
the defaults for maxThreads (max-connection option in config file) are described at [4], so it should be:
512 * number of processors in engine server
[4] https://access.redhat.com/solutions/25054