RFC Error: Logon failed. See System Log (transaction SM21) for details.

What causes this issue?

A Remote Function Call (RFC) to backend system terminated with a system
failure exception.

System Response

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

How to fix this error?

In this case a very generic error text "Logon failed ..." is returned by
the RFC framework. An analysis of System Log (transaction SM21) or
Development Traces (transaction ST11) is needed to find out the root
cause.
Contact SAP Support if you can't solve this issue.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.