Unable to save to 'field_title' for page that doesn't exist in pages table

[Jozsef]

I'm creating a new page from a form submission. It has been working perfectly but now it fails to save the new page with the error message:

Error saving field "title" - Unable to save to 'field_title' for page that doesn't exist in pages table

Can any of you spot any error in my code? It's directly in a template file.
I probably just can't see the wood for the trees after hours of searching.
The funny thing is that I haven't changed anything in that part of the template.

   // create the page
    $order = new Page();
    // Set the template
    $order->template = $templates->get("order");
    // Set the parent page
    $order->parent = $pages->get("/orders/");

    // Set the title/name field
    $order->title = $event_title . " - " . $u->first_name . " " . $u->last_name;
    // $order->headline = $headline;
    $order->order_event = $event;
    $order->order_user  = $u->id;
    $order->quantity = $qty;
    $order->price = $price;
    // Calculate and save total
    $order->total = $total;
    $order->handling = $handling;
    $order->balance = $balance;

    // save the order
    $order->of(false);
    $order->save();

Update:

New pages don't save from the backend either, giving the same error message for each field as admin notifications.