diff --git a/app/models/calendar.rb b/app/models/calendar.rb
index dca9048ec..c3ed16db0 100644
--- a/app/models/calendar.rb
+++ b/app/models/calendar.rb
@@ -8,11 +8,14 @@ class Calendar < ApplicationModel
   store :business_hours
   store :public_holidays
 
-  before_create  :validate_public_holidays, :validate_hours, :fetch_ical
-  after_create   :sync_default, :min_one_check
-  before_update  :validate_public_holidays, :validate_hours, :fetch_ical
-  after_update   :sync_default, :min_one_check
-  after_destroy  :min_one_check
+  validate :validate_hours
+
+  before_save :ensure_public_holidays_details, :fetch_ical
+
+  after_destroy :min_one_check
+  after_save    :min_one_check
+
+  after_save :sync_default
 
 =begin
 
@@ -391,8 +394,8 @@ returns
     true
   end
 
-  # validate format of public holidays
-  def validate_public_holidays
+  # ensure integrity of details of public holidays
+  def ensure_public_holidays_details
 
     # fillup feed info
     before = public_holidays_was
@@ -414,7 +417,13 @@ returns
 
     # get business hours
     hours = business_hours_to_hash
-    raise Exceptions::UnprocessableEntity, 'No configured business hours found!' if hours.blank?
+
+    if hours.blank?
+      errors.add :base, 'No configured business hours found!'
+      return
+    end
+
+    #raise Exceptions::UnprocessableEntity, 'No configured business hours found!' if hours.blank?
 
     # validate if business hours are usable by execute a try calculation
     begin
@@ -423,10 +432,8 @@ returns
       end
       Biz.time(10, :minutes).after(Time.zone.parse('Tue, 05 Feb 2019 21:40:28 UTC +00:00'))
     rescue => e
-      raise Exceptions::UnprocessableEntity, e.message
+      errors.add :base, e.message
     end
-
-    true
   end
 
 end
diff --git a/db/migrate/20210617051913_issue_3618_google_calendar_url_https.rb b/db/migrate/20210617051913_issue_3618_google_calendar_url_https.rb
index 2d53227ed..b1e472a8a 100644
--- a/db/migrate/20210617051913_issue_3618_google_calendar_url_https.rb
+++ b/db/migrate/20210617051913_issue_3618_google_calendar_url_https.rb
@@ -7,8 +7,10 @@ class Issue3618GoogleCalendarUrlHttps < ActiveRecord::Migration[5.2]
     Calendar
       .where('ical_url LIKE ?', 'http://www.google.com/calendar/ical/%')
       .each do |calendar|
-        calendar.ical_url.sub!(%r{^http://}, 'https://')
-        calendar.save
+        new_url = calendar.ical_url.sub(%r{^http://}, 'https://')
+        # skipping validation allows to update old misconfigured calendar
+        # https://github.com/zammad/zammad/issues/3641
+        calendar.update_attribute :ical_url, new_url # rubocop:disable Rails/SkipsModelValidations
       end
   end
 end
diff --git a/spec/db/migrate/issue_3618_google_calendar_url_https_spec.rb b/spec/db/migrate/issue_3618_google_calendar_url_https_spec.rb
index 05f526855..d73391c6b 100644
--- a/spec/db/migrate/issue_3618_google_calendar_url_https_spec.rb
+++ b/spec/db/migrate/issue_3618_google_calendar_url_https_spec.rb
@@ -12,4 +12,13 @@ RSpec.describe Issue3618GoogleCalendarUrlHttps, type: :db_migration, db_strategy
       .from(false)
       .to(true)
   end
+
+  it 'other' do
+    calendar.update_attribute(:business_hours, nil)
+
+    expect { migrate }
+      .to change { calendar.reload.ical_url.starts_with? 'https://' }
+      .from(false)
+      .to(true)
+  end
 end
diff --git a/spec/models/calendar_spec.rb b/spec/models/calendar_spec.rb
index 418c6c95d..f2f66cab8 100644
--- a/spec/models/calendar_spec.rb
+++ b/spec/models/calendar_spec.rb
@@ -211,14 +211,14 @@ RSpec.describe Calendar, type: :model do
                      timeframes: [['09:00', '00:00']]
                    }
                  })
-        end.to raise_error(Exceptions::UnprocessableEntity, 'nonsensical hours provided')
+        end.to raise_error(ActiveRecord::RecordInvalid, %r{nonsensical hours provided})
       end
 
       it 'fails for blank structure' do
         expect do
           create(:calendar,
                  business_hours: {})
-        end.to raise_error(Exceptions::UnprocessableEntity, 'No configured business hours found!')
+        end.to raise_error(ActiveRecord::RecordInvalid, %r{No configured business hours found!})
       end
     end
   end