Appian upgrade results in 403 Errors seen on the user interface with "com.appiancorp.kougar.driver.exceptions.Signal: Cannot call itself" error seen in the application server log

Symptoms

Users intermittently observe a 403 error on the interface.

An error of the following type is observed in the server log at the same time:

[Appian Work Item - 341 - ProcessExec00 : CatchupRequest] ERROR com.appiancorp.process.engine.CatchupRequest - Failed to register catchup (0), can retry
INFO [stdout] (Appian Work Item - 341 - ProcessExec00 : CatchupRequest) com.appiancorp.kougar.driver.exceptions.Signal: Cannot call itself
INFO [stdout] (Appian Work Item - 341 - ProcessExec00 : CatchupRequest) at com.appiancorp.kougar.driver.ipc.IpcConnection.get(IpcConnection.java:390)

db_PX** logs for the Appian Execution engines, has an error of the following type:
[PX011] {pe975.kdb ,1548295} (Default) WARN .a.p.PROCESS.i "Incremental Update: Exec Engine is not ready to start Incremental update.

Cause

The errors indicate that there was an issue with the engine migration. It's possible that the engines were not shutdown gracefully before migrating them. 

Action

Perform the migration again as documented here and make sure the engines are stopped gracefully on the original environment.

Affected Versions

This article applies to all versions of Appian.