diff --git a/src/pretix/base/models.py b/src/pretix/base/models.py
index 35cc908add..b0169acb31 100644
--- a/src/pretix/base/models.py
+++ b/src/pretix/base/models.py
@@ -1559,8 +1559,6 @@ class Order(Versionable):
     def _is_still_available(self):
         error_messages = {
             'unavailable': _('Some of the ordered products were no longer available.'),
-            'busy': _('We were not able to process the request completely as the '
-                      'server was too busy.'),
         }
         positions = list(self.positions.all().select_related(
             'item', 'variation'
@@ -1570,31 +1568,26 @@ class Order(Versionable):
         ))
         quota_cache = {}
         try:
-            with self.event.lock():
-                for i, op in enumerate(positions):
-                    quotas = list(op.item.quotas.all()) if op.variation is None else list(op.variation.quotas.all())
-                    if len(quotas) == 0:
-                        raise Quota.QuotaExceededException(error_messages['unavailable'])
+            for i, op in enumerate(positions):
+                quotas = list(op.item.quotas.all()) if op.variation is None else list(op.variation.quotas.all())
+                if len(quotas) == 0:
+                    raise Quota.QuotaExceededException(error_messages['unavailable'])
 
-                    for quota in quotas:
-                        # Lock the quota, so no other thread is allowed to perform sales covered by this
-                        # quota while we're doing so.
-                        if quota.identity not in quota_cache:
-                            quota_cache[quota.identity] = quota
-                            quota.cached_availability = quota.availability()[1]
-                        else:
-                            # Use cached version
-                            quota = quota_cache[quota.identity]
-                        quota.cached_availability -= 1
-                        if quota.cached_availability < 0:
-                            # This quota is sold out/currently unavailable, so do not sell this at all
-                            raise Quota.QuotaExceededException(error_messages['unavailable'])
+                for quota in quotas:
+                    # Lock the quota, so no other thread is allowed to perform sales covered by this
+                    # quota while we're doing so.
+                    if quota.identity not in quota_cache:
+                        quota_cache[quota.identity] = quota
+                        quota.cached_availability = quota.availability()[1]
+                    else:
+                        # Use cached version
+                        quota = quota_cache[quota.identity]
+                    quota.cached_availability -= 1
+                    if quota.cached_availability < 0:
+                        # This quota is sold out/currently unavailable, so do not sell this at all
+                        raise Quota.QuotaExceededException(error_messages['unavailable'])
         except Quota.QuotaExceededException as e:
             return str(e)
-        except EventLock.LockTimeoutException:
-            # Is raised when there are too many threads asking for quota locks and we were
-            # unaible to get one
-            return error_messages['busy']
         return True
 
     @property
diff --git a/src/pretix/base/services/locking.py b/src/pretix/base/services/locking.py
index b87acc3470..bdc8c5a0db 100644
--- a/src/pretix/base/services/locking.py
+++ b/src/pretix/base/services/locking.py
@@ -52,7 +52,7 @@ def release_event(event):
     :raises EventLock.LockReleaseException: if we do not own the lock
     """
     if not hasattr(event, '_lock') or not event._lock:
-        raise EventLock.LockReleaseException('')
+        raise EventLock.LockReleaseException('Lock is not owned by this thread')
     if settings.HAS_REDIS:
         return release_event_redis(event)
     else:
diff --git a/src/pretix/control/views/orders.py b/src/pretix/control/views/orders.py
index 7fbb9d2d10..d9f97e917e 100644
--- a/src/pretix/control/views/orders.py
+++ b/src/pretix/control/views/orders.py
@@ -12,7 +12,9 @@ from django.utils.timezone import now
 from django.utils.translation import ugettext_lazy as _
 from django.views.generic import DetailView, ListView, TemplateView, View
 
-from pretix.base.models import CachedFile, CachedTicket, Item, Order, Quota
+from pretix.base.models import (
+    CachedFile, CachedTicket, EventLock, Item, Order, Quota,
+)
 from pretix.base.services.export import export
 from pretix.base.services.orders import mark_order_paid
 from pretix.base.services.stats import order_overview
@@ -247,12 +249,17 @@ class OrderExtend(OrderView):
             if oldvalue > now():
                 self.form.save()
             else:
-                is_available = self.order._is_still_available()
-                if is_available is True:
-                    self.form.save()
-                    messages.success(self.request, _('The payment term has been changed.'))
-                else:
-                    messages.error(self.request, is_available)
+                try:
+                    with self.order.event.lock():
+                        is_available = self.order._is_still_available()
+                        if is_available is True:
+                            self.form.save()
+                            messages.success(self.request, _('The payment term has been changed.'))
+                        else:
+                            messages.error(self.request, is_available)
+                except EventLock.LockTimeoutException:
+                    messages.error(self.request, _('We were not able to process the request completely as the '
+                                                   'server was too busy.'))
             return self._redirect_back()
         else:
             return self.get(*args, **kwargs)