The error log line "Connector" contains more valuable information. The client error only knows something happened. not what.
Can you share the connector log as well?
you can also check my comments below, thank you
The error message states it occurs in SendEmail_2 at the SendEmail java action. The error itself is a NullPointer, which looks to refer to the fact that an object required for the action does not exist. You can use a debugger to see what objects exist exactly before the action is executed and then work out why it is not there.