Actually, when I tried something more.
When I use: Filter(Salesorders, SalesOrderNumber,"SO-1000") it works fine.
When I use: Filter(Salesorders, ProjectNumber,"Proj-1000") it gives a fault
When I use: Filter(Salesorders, SalesOrderNumber,RecordsGallery1.Selected.SalesOrderNumber) it works fine
When I use: Filter(Salesorders, SalesOrderNumber,RecordsGallery1.Selected.ProjectNumber) it gives a fault.
It looks like the ProjectNumber does't exist in the SalesOrder table. Because When I make a dropdown list, with items 'SalesOrder' I cannot have the value ProjectNumber. It looks like it is because it is a LookUp column, does that make sense? And how to solve?
Thanks