Internal error: unknown exception occurred

What causes this issue?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

Processing terminated.

How to fix this error?

INCLUDE ISR_SYSTEMFEHLER
Note for system administration:
Function module &V1& triggered an exception. This exception is unknown
in the calling function module/program &V2& and could not be processed.

Error message extract from SAP system. Copyright SAP SE.