Shield undo you latest changes and bring it back to the state that previously worked.
That error message looks like a rule issue. For example if you execute “update ticket tag” action inside “ticket tag updated” rule that will be an infinite loop because it infinitely updates ticket tag and it infinitely executes “ticket tag updated” rule.
Very common for people to think this would work. SambaPOS data is not stored in the software. It is stored in a Database. So deleting the software and reinstalling does nothing. Trust me it is a good thing it works this way.
So if you can not remember what you did and the problem still persists you have a few options. If you do not have any important data you could delete the database and create a new one. You could backup your database then delete it and create a new one.
You could try to remember the changes you made and revert back.
Or you can give us more information of what you changed and provide screenshots of those rules. Without more information I am sorry we just can not know exactly what to tell you to do.
EDIT: OR If your comfortable with it you could backup your database and PM me a copy I can take a look at it.