After 5000 items threshold is exceeded, custom new form stops working even though I did index a couple of columns, and views are working fine, so as default the new form. But when I want to open the custom new form, I get an error.
Read somewhere it might be a problem with the query which should be changed, but I can't find a concrete explanation or example.
This view cannot be displayed because it exceeds the list view threshold (5000 items) enforced by the administrator.
To view items, try selecting another view or creating a new view. If you do not have sufficient permissions to create views for this list, ask your administrator to modify the view so that it conforms to the list view threshold.