A concurrent request was running into an error. The following error was shown in the logfile of the request.
APP-FND-00362: Routine afpbep cannot execute request &REQUEST for program &PROGRAM, because the environment variable &BASEPATH is not set for the application to which the concurrent program executable &EXECUTABLE belongs.
I noticed the $CUSTOM_TOP variable was not set, but after setting this variable the same error occurred.
Then I noticed something strange, the $XXX_TOP was not set in the environment. After adding this top to the adovars.env (to test the request), the concurrent request ended without error.