D365 Retail MPOS login error "The request passed to the service is not valid"

Dynamics 365 for Retail error that occurs when attempting to log in to the Modern POS (MPOS): "The request passed to the service is not valid"

This occurs when an existing shift is trying to be opened by the MPOS but it cannot.

This issue is usually because the user’s shift is open on another terminal and the user can only log into a single shift.

To recreate this, have two POS terminals for the same store. Open a new shift on terminal 1. Then, while the first shift is open, log in to the second shift. If you have your system configured to allow a single user, single shift, it will ask you to resume an existing shift, the one that you have open on terminal 1. The system will give you the error “The request passed to the service is not valid”.

This can also occur if you find yourself in a ‘stuck shift’ issue. That can happen when your system allows for shared shift drawers (hardware profile) and you are switching terminals. I am not sure in what order that you’ll have to do things to receive this ‘stuck shift’ issue, but it will result in your user, and the others who were using that shared shift that is now stuck, to receive that “the request passed…” error. Those users on that essentially hose them from the system. They can’t log in to any POS terminal for that store. Not fun...

For this stuck shift issue, look at the crt.RetailShiftUsersStagingTable and crt.RetailShiftStagingTable tables in SQL Express for the stuck shifts. You may need to do some magic here. Be extremely careful in what you do to fix the issue. While our team was able to resolve the issue, you may want to use Microsoft Support to assist with the issue. Shout out to the team for figuring out this issue! 

UPDATE 6/12/2018 - Check out my buddy Duc Le's blog post about this very issue: http://ducdax2012retail.blogspot.com/2017/02/cant-close-shift-error.html. Duc was a colleague of mine a while back so check out his blog! He focused on retail as well. Thanks for the comment below and miss working with you, dude. Hope all has been well!