No proxy class could be determined for proxy interface &1

What causes this issue?

An inconsistency was detected with the ABAP proxy: to a give proxy
interface no proxy class could be determined.

System Response

The current action is aborted with an exception message.
If the action is the creation of a new project, the project will not be
established.

How to fix this error?

Go to the ABAP proxy (transaction SPROXY) and check it.
Generate the proxy if necessary.
Activate the proxy if is not active.
Verify that the proxy class is specified and active.
Verify that no other classes exist that implement the proxy interface
(e.g. after a replacement of the proxy class the former class must be
deleted).

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.