diff --git a/lib/api/errors/error_base.rb b/lib/api/errors/error_base.rb
index 88d7f65565..1b11b48efa 100644
--- a/lib/api/errors/error_base.rb
+++ b/lib/api/errors/error_base.rb
@@ -33,9 +33,11 @@ module API
       attr_reader :code, :message, :details, :errors
 
       def self.create(errors)
-        [:error_unauthorized, :error_conflict, :error_readonly].each do |key|
+        [:error_not_found, :error_unauthorized, :error_conflict, :error_readonly].each do |key|
           if errors.has_key?(key)
             case key
+            when :error_not_found
+              return ::API::Errors::NotFound.new(errors[key].join(' '))
             when :error_unauthorized
               return ::API::Errors::Unauthorized
             when :error_conflict
diff --git a/lib/api/v3/work_packages/form/form_api.rb b/lib/api/v3/work_packages/form/form_api.rb
index 9a7cef6709..2b88a3aa97 100644
--- a/lib/api/v3/work_packages/form/form_api.rb
+++ b/lib/api/v3/work_packages/form/form_api.rb
@@ -31,8 +31,33 @@ module API
     module WorkPackages
       module Form
         class FormAPI < Grape::API
+          helpers do
+            def process_form_request
+              if form_post_request_body
+                # enforces availibility validation of lock_version
+                @representer.represented.lock_version = nil
+                @representer.from_json(patch_request_body)
+
+                patch_request_valid?
+              end
+
+              error = ::API::Errors::ErrorBase.create(@representer.represented.errors)
+
+              if error.is_a? ::API::Errors::Validation
+                status 200
+                FormRepresenter.new(@representer.represented, current_user: current_user)
+              else
+                fail error
+              end
+            end
+
+            def form_post_request_body
+              env['api.request.body']
+            end
+          end
+
           post '/form' do
-            FormRepresenter.new(@work_package, current_user: current_user)
+            process_form_request
           end
         end
       end
diff --git a/lib/api/v3/work_packages/work_package_contract.rb b/lib/api/v3/work_packages/work_package_contract.rb
index 6189d8115d..37392f7a4d 100644
--- a/lib/api/v3/work_packages/work_package_contract.rb
+++ b/lib/api/v3/work_packages/work_package_contract.rb
@@ -43,9 +43,10 @@ module API
           @can = WorkPackagePolicy.new(user)
         end
 
+        validate :user_allowed_to_access
         validate :user_allowed_to_edit
         validate :user_allowed_to_edit_parent
-        validate :lock_version_set
+        validate :lock_version_valid
         validate :readonly_attributes_unchanged
 
         extend Reform::Form::ActiveModel::ModelValidations
@@ -53,6 +54,13 @@ module API
 
         private
 
+        def user_allowed_to_access
+          unless ::WorkPackage.visible(@user).exists?(model)
+            message = "Couldn't find WorkPackage with id=#{model.id}"
+            errors.add :error_not_found, message
+          end
+        end
+
         def user_allowed_to_edit
           errors.add :error_unauthorized, '' unless @can.allowed?(model, :edit)
         end
@@ -63,8 +71,8 @@ module API
           end
         end
 
-        def lock_version_set
-          errors.add :error_conflict, '' if model.lock_version.nil?
+        def lock_version_valid
+          errors.add :error_conflict, '' if model.lock_version.nil? || model.lock_version_changed?
         end
 
         def readonly_attributes_unchanged