Sort order of views pane not saving

I have several views panes with exposed sort order.
I clicked "Use exposed widgets form as pane configuration" in "Allow settings" in the view configuration
The sort order select box is appearing as expected. But the selection is never saved. The top sort order of the view is always executed.
In the database I find the entry 's:7:"sort_by";s:7:"created"' (table: 'panels_pane', column: 'configuration')
even if the shown and executed sort order is a different one. When changing the entry in the database to 's:7:"sort_by";s:7:"last_updated"' I get a server error.