Exception & raised; no application servers found

What causes this issue?

The message server cannot be reached or does not give a list of
application servers available. The most common reason for this exception
is the missing message server. In the working directory of the SAP
system, you can find the trace file "dev_ms".

System Response

The upgrade stops with an error message.

How to fix this error?

Start the message server again, by restarting the SAP system, for
example. Using the test environment of the function module
TH_SERVER_LIST, check that the message server is running correctly.

Error message extract from SAP system. Copyright SAP SE.