Hi all. Thanks for all advices. The problem was really that my grid was called from ref set selector. I have used XPath constraint in Selectable objects property and it is working now.
Have a nice day.
Lukas
It's difficult to be certain without seeing your domain model, but it looks to me like your X-path may have gone down too many levels.
Try
[Staff.Staff_Company/Customers.Company/Customers.RegisteredUser_Company = '[%CurrentUser%]']
This one is checking that (reading from right to left) the current user is a registered user linked to a company, and the company is linked to the Staff to be displayed, which should be all that you need.