Error Setting Sign on PeopleCode context for User

The first sign you might have this error is if you see the following message on the sign on screen:

You can usually still login and use the application. What tweaked me to a deeper problem was that the web profile I expected (DEV) was not being used, and so the CTRL+J option was not available. On checking the application server logs, I found the following messages (formatted for readability):

Error Setting Sign on PeopleCode context for User @MACHINE: Sign on PeopleCode was not executed
PeopleSoft ID and Password authentication failed. Invalid user {V1.1}JP9ukEkTssmYrzsK1yvXFg==@MACHINE.
{V1.1}JP9ukEkTssmYrzsK1yvXFg==@MACHINE is an Invalid User ID, or you typed the wrong password.  
User ID and Password are required and case-sensitive. Make sure you're typing in the correct upper and lower case.  
Failed to execute GetCertificate request

First I checked there was nothing wrong with the encrypted user PTWEBSERVER, ensuring the correct password and encrypted password in configuration.properties, the account was unlocked, and they had the role PeopleTools Web Server. In fact nothing had changed at all in terms of web server configuration.

Eventually, I found that the windows firewall might have been blocking the java process for the web server. Since the web server was being started by a service, this was not evident. I only realised the problem after stopping the service and then trying to manually start it with the startPIA.bat file under PS_HOME\webserv\{DOMAIN}\bin\startPIA.bat. When I did this, it brought up the windows firewall dialog asking to allow private/public network access for Java which I accepted.

I then stopped the PIA with the stopPIA.bat file (in the same folder), and then started the PIA using the service. However the problem came back. So I uninstalled the service using PS_HOME\webserv\{DOMAIN}\bin\uninstallNTServicePIA.cmd and then installed it again using PS_HOME\webserv\{DOMAIN}\bin\installNTServicePIA.cmd.

This fixed the issue. Perhaps it was an issue with the service all along? If that's the case, then simply uninstalling and re-installing the service might be all that you need to do to fix this.