This list will be updated regularly as IBMI AS/400 software issues arise. Please feel free to add comments or your own experiences with errors. Please keep in mind OS/400 is typically paired with 3rd party applications and some errors may contain the subsystems involved.
Error code:
Job User/Job# started on DATE at TIME in subsystem
Message queue User is allocated to another job.
Error message CPF4169 appeared during OPEN for file (FILENAME) (C S D F).

This error is very common, especially with a fresh or updated AS/400 Install. This can be fixed very simply by adjusting the screen size settings. Originally AS/400 was used on much smaller screens, so the default configs (even on new versions of IBMI) are set to 24x80.
To fix: Go to top and click Communication > Configure > Screen size
Should be a drop down to allow you to change from 24x80 to 27x132
* Some emulators aren't the exact same flow so try and find the display settings. Just dig around to it, I use 3 different emulators and they are all pretty easy to find so just keep looking.
List of FTP IP addresses Associated With Your Machine
In the event you need to tighten up your firewall, or just want a better scope of FTP's on your machine. You can easily access this list by typing CMD: 'WRKCNN' for Work connection. This will give you a full list of IP's and type - From there you can filter your search to Type F (FTP) and get a full list.

I crossed out our info but your screen will display a full list of what's attached.
User Access Denied or Not Authorized Errors
Sometimes users will only have limited or no access to either Commands, Menu's or even parts of menus. Subsystems can cause havoc when dealing with these issue but it's safe to say a good place to start is to see if the user even has the ability to see certain menus.

What I usually do is log onto an administrator account or SECOFR account and run CMD: EDTOBJAUT - This will let you search for a large amount of objects identified in your machine such as *CMD or *USRPRF (But there are about 65 options just hit F4 for a list)
In this example I will use a *CMD restriction

So in this example I am looking at what users have access to the command "Go Secure" which is a subsystem command.

When I hit enter you can see that all public users have access to this command. If I wanted to isolate a user I can hit F6 to add a user and assign it *EXCLUDE to prohibit them from using the command. Note: In most systems this doesn't grant access to assign EDTOBJAUT commands to anyone so although you might be successful in navigating the menu - you may not be able to actually assign the rule if your account doesn't have those permissions.
Putty Telnet connection to AS/400
To access your AS/400 machine, you can optionally open a Putty session using port 23. Just plug in your Host address as your machine IP connect. It's the same type of connection as an emulator.

Telnet can be a great solution to