Send E-mail smart service breaks with "An error occurred in executing an activity class." However, when I restart the node, it works fine.

We have a wrapper process(Process A) which calls another process(Process B) as an MNI. This process B has a send e-mail smart service node. We have observed that this send e-mail smart service node breaks in about 25% of the cases and the alert simply says "An error occurred in executing an activity class." 

However, when I go into the process and re-start the broken node without making any changes, then the node works fine. Has anyone else encountered this? What is the solution to avoid it?

Thank you!