UE4 & Logging - 500 error but get email

I created a test form, submitted it, received all emails (to, from, cc), and the record wrote to the database.

I then added the functionality to a different form on the same domain. The emails were sent/received, but I got a page that says "HTTP Error 500.0 - Internal Server Error", and no data was written to the database.

So I tried the original test form to see if maybe I had done something wrong in creating this new form, but this form gave the same result as the second form - email sent, 500 error.

This is above my skillset, but my sister (who teaches programming) says it sounds like something that happens with her students a lot - which is, they didn't "close" the connection to the database after writing the record (? - did I say that right?) Could I have done something in my testing that made that happen?

I am an administrator on our webserver, so I set up Failed Request Tracing, and have a logs that you can see if it will help. I only see a warning for event "MODULE_SET_RESPONSE_ERROR_STATUS" in FastCgiModule

It says:

ModuleName: FastCgiModule
Notification: 128
HttpStatus: 500
HttpReason: Internal Server Error
HttpSubStatus: 0
ErrorCode: 0
ConfigExceptionInfo:
Notification: EXECUTE_REQUEST_HANDLER
ErrorCode: The operation completed successfully. (0x0)

There is no other error or warning in the rest of the log.

Can you help? Or do you need more info / screenshots / whatever...?

As always, thank you in advance for your help!

~J