When using the API (through the sandbox to eliminate client side issues) for System_SearchEvents and loading an example to the editor and replacing RefID with a genuine InvoiceId (or just leaving the auto populated one which is not a real InvoiceId for my account, the API is returning an unhandled error.
{
"Errors": {
"Error": [
"Unhandled error occurred"
]
}
}
This was working recently when I tested it.