User switch is not possible (SY-SUBRC = &)

What causes this issue?

Asynchronous transactional RFCs (tRFC/qRFC inbound and outbound) can
only be started in the ABAP systems in which they were created.
Units which were created on ABAP system A and then copied to ABAP system
B can no longer be executed. This problem may occur when copying systems
with database resources.

How to fix this error?

The LUWs which can no longer be executed must be deleted.

Procedure for System Administrators

You can delete them by using the monitor program of the tRFC (SM58),
qRFC outbound (SMQ1), and qRFC inbound (SMQ2) monitors.

Error message extract from SAP system. Copyright SAP SE.