Fixed serialization issue

src/pretix/control/forms/orders.py

@@ -1,3 +1,6 @@
+from django import forms
+from django.db import models
+
 from pretix.base.forms import I18nModelForm
 from pretix.base.models import Order
 
@@ -6,3 +9,17 @@ class ExtendForm(I18nModelForm):
     class Meta:
         model = Order
         fields = ['expires']
+
+
+class ExporterForm(forms.Form):
+
+    def clean(self):
+        data = super().clean()
+
+        for k, v in data.items():
+            if isinstance(v, models.Model):
+                data[k] = v.pk
+            elif isinstance(v, models.QuerySet):
+                data[k] = [m.pk for m in v]
+
+        return data

src/pretix/control/views/orders.py

@@ -26,7 +26,7 @@ from pretix.base.signals import (
     register_data_exporters, register_payment_providers,
     register_ticket_outputs,
 )
-from pretix.control.forms.orders import ExtendForm
+from pretix.control.forms.orders import ExporterForm, ExtendForm
 from pretix.control.permissions import EventPermissionRequiredMixin
 from pretix.multidomain.urlreverse import build_absolute_uri
 
@@ -391,7 +391,7 @@ class ExportView(EventPermissionRequiredMixin, TemplateView):
         responses = register_data_exporters.send(self.request.event)
         for receiver, response in responses:
             ex = response(self.request.event)
-            ex.form = forms.Form(
+            ex.form = ExporterForm(
                 data=(self.request.POST if self.request.method == 'POST' else None)
             )
             ex.form.fields = ex.export_form_fields