diff --git a/app/domain/invoicing/small_invoice/address_sync.rb b/app/domain/invoicing/small_invoice/address_sync.rb
index 74707f36..0d160d18 100644
--- a/app/domain/invoicing/small_invoice/address_sync.rb
+++ b/app/domain/invoicing/small_invoice/address_sync.rb
@@ -40,6 +40,7 @@ def record_to_params(record, prefix = 'billing_address')
 
       delegate :notify_sync_error, to: 'self.class'
       attr_reader :client, :remote_keys
+
       class_attribute :rate_limiter
       self.rate_limiter = RateLimiter.new(Settings.small_invoice.request_rate)
 
@@ -49,13 +50,16 @@ def initialize(client, remote_keys = nil)
       end
 
       def sync
+        failed = []
         ::BillingAddress.includes(:client).where(client_id: client.id).find_each do |billing_address|
           key(billing_address) ? update_remote(billing_address) : create_remote(billing_address)
         rescue StandardError => e
+          failed << billing_address.id
           Rails.logger.error e.message
           Rails.logger.error e.backtrace
           notify_sync_error(e, billing_address)
         end
+        Rails.logger.error "Failed Address Syncs: #{failed.inspect}" if failed.any?
       end
 
       private
diff --git a/app/domain/invoicing/small_invoice/api.rb b/app/domain/invoicing/small_invoice/api.rb
index 484d0714..645b1b88 100644
--- a/app/domain/invoicing/small_invoice/api.rb
+++ b/app/domain/invoicing/small_invoice/api.rb
@@ -50,7 +50,7 @@ def get_raw(path, auth: true, **params)
 
       def access_token
         # fetch a new token if we have none yet or if the existing one is expired
-        @access_token, @expires_at = get_access_token unless @expires_at &. > Time.zone.now
+        @access_token, @expires_at = get_access_token unless @expires_at&.>(Time.zone.now)
         @access_token
       end
 
diff --git a/app/domain/invoicing/small_invoice/client_sync.rb b/app/domain/invoicing/small_invoice/client_sync.rb
index a9e75d6d..11138e8a 100644
--- a/app/domain/invoicing/small_invoice/client_sync.rb
+++ b/app/domain/invoicing/small_invoice/client_sync.rb
@@ -10,15 +10,18 @@ class ClientSync
       class << self
         def perform
           remote_keys = fetch_remote_keys
+          failed = []
           ::Client.includes(:work_item, :contacts, :billing_addresses).find_each do |client|
             if client.billing_addresses.present? # required by small invoice
               begin
                 new(client, remote_keys).sync
               rescue => error
+                failed << client.id
                 notify_sync_error(error, client)
               end
             end
           end
+          Rails.logger.error "Failed Client Syncs: #{failed.inspect}" if failed.any?
         end
 
         def fetch_remote_keys
@@ -60,6 +63,7 @@ def record_to_params(record, prefix = 'client')
 
       delegate :notify_sync_error, to: 'self.class'
       attr_reader :client, :remote_keys
+
       class_attribute :rate_limiter
       self.rate_limiter = RateLimiter.new(Settings.small_invoice.request_rate)
 
diff --git a/app/domain/invoicing/small_invoice/contact_sync.rb b/app/domain/invoicing/small_invoice/contact_sync.rb
index 26f83c89..d8020c10 100644
--- a/app/domain/invoicing/small_invoice/contact_sync.rb
+++ b/app/domain/invoicing/small_invoice/contact_sync.rb
@@ -40,6 +40,7 @@ def record_to_params(record, prefix = 'billing_address')
 
       delegate :notify_sync_error, to: 'self.class'
       attr_reader :client, :remote_keys
+
       class_attribute :rate_limiter
       self.rate_limiter = RateLimiter.new(Settings.small_invoice.request_rate)
 
@@ -49,11 +50,14 @@ def initialize(client, remote_keys = nil)
       end
 
       def sync
+        failed = []
         ::Contact.includes(:client).where(client_id: client.id).find_each do |contact|
           key(contact) ? update_remote(contact) : create_remote(contact)
         rescue StandardError => e
+          failed << contact.id
           notify_sync_error(e, contact)
         end
+        Rails.logger.error "Failed Contact Syncs: #{failed.inspect}" if failed.any?
       end
 
       private
diff --git a/app/domain/invoicing/small_invoice/invoice_sync.rb b/app/domain/invoicing/small_invoice/invoice_sync.rb
index 850f4c39..0ef70b88 100644
--- a/app/domain/invoicing/small_invoice/invoice_sync.rb
+++ b/app/domain/invoicing/small_invoice/invoice_sync.rb
@@ -25,18 +25,22 @@ class InvoiceSync
       }.freeze
 
       attr_reader :invoice
+
       class_attribute :rate_limiter
       self.rate_limiter = RateLimiter.new(Settings.small_invoice.request_rate)
 
       class << self
         def sync_unpaid
+          failed = []
           unpaid_invoices.find_each do |invoice|
             begin
               new(invoice).sync
             rescue => error
+              failed << invoice.id
               notify_sync_error(error, invoice)
             end
           end
+          Rails.logger.error "Failed Invoice Syncs: #{failed.inspect}" if failed.any?
         end
 
         private
diff --git a/app/models/invoice.rb b/app/models/invoice.rb
index 094333fa..94cf0017 100644
--- a/app/models/invoice.rb
+++ b/app/models/invoice.rb
@@ -49,12 +49,12 @@ class Invoice < ActiveRecord::Base
   before_validation :generate_reference, on: :create
   before_validation :generate_due_date
   before_validation :update_totals
-  before_create :lock_client_invoice_number
-  after_create :update_client_invoice_number
-  after_save :update_order_billing_address
   before_save :save_remote_invoice, if: -> { Invoicing.instance.present? }
   before_save :assign_worktimes
+  before_create :lock_client_invoice_number
+  after_create :update_client_invoice_number
   after_destroy :delete_remote_invoice, if: -> { Invoicing.instance.present? }
+  after_save :update_order_billing_address
 
   protect_if :paid?, 'Bezahlte Rechnungen können nicht gelöscht werden.'
   protect_if :order_closed?, 'Rechnungen von geschlossenen Aufträgen können nicht gelöscht werden.'
@@ -225,7 +225,11 @@ def save_remote_invoice
     self.invoicing_key = Invoicing.instance.save_invoice(self, positions)
   rescue Invoicing::Error => e
     errors.add(:base, "Fehler im Invoicing Service: #{e.message}")
-    Rails.logger.error(e.class.name + ': ' + e.message + "\n" + e.data.inspect + "\n" + e.backtrace.join("\n"))
+    Rails.logger.error <<~ERROR
+      #{e.class.name}: #{e.message}
+      #{e.data.inspect}
+      #{e.backtrace.join("\n")}
+    ERROR
     throw :abort
   end