diff --git a/lib/orientdb4r.rb b/lib/orientdb4r.rb
index 06e6d65..de32ca5 100644
--- a/lib/orientdb4r.rb
+++ b/lib/orientdb4r.rb
@@ -27,6 +27,7 @@ module Binary
   autoload :RoundRobin,       'orientdb4r/load_balancing'
   autoload :DocumentMetadata, 'orientdb4r/rest/model'
 
+  MUTEX_CLIENT = Mutex.new
 
   class << self
 
@@ -42,7 +43,7 @@ def client options={}
         return options.delete(:binary) ? Binary::BinClient.new(options) : RestClient.new(options)
       end
 
-      Thread.exclusive {
+      MUTEX_CLIENT.synchronize {
         client = options.delete(:binary) ? Binary::BinClient.new(options) : RestClient.new(options)
         Thread.current[:orientdb_client] ||= client
         #Thread.current[:orientdb_client] ||= BinClient.new options
@@ -62,10 +63,18 @@ class OrientdbError < StandardError
     include ChainedError
   end
 
+  ###
+  # Error indicating that the request is malformed of invalid somehow.
+  class InvalidRequestError < OrientdbError; end
+
   ###
   # Error indicating that access to the resource requires user authentication.
   class UnauthorizedError < OrientdbError; end
 
+  ###
+  # Error indicating a conflict between database state and the request
+  class StateConflictError < OrientdbError; end
+
   ###
   # Error indicating that the server encountered an unexpected condition which prevented it from fulfilling the request.
   class ServerError < OrientdbError; end
diff --git a/lib/orientdb4r/rest/client.rb b/lib/orientdb4r/rest/client.rb
index 9a2731e..14cbf90 100644
--- a/lib/orientdb4r/rest/client.rb
+++ b/lib/orientdb4r/rest/client.rb
@@ -331,9 +331,7 @@ def get_class(name) #:nodoc:
       raise ArgumentError, "class name is blank" if blank?(name)
 
       response = call_server(:method => :get, :uri => "class/#{@database}/#{name}")
-      rslt = process_response(response) do
-        raise NotFoundError, 'class not found' if response.body =~ /Invalid class/
-      end
+      rslt = process_response(response)
 
       classes = [rslt]
       decorate_classes_with_model(classes)
@@ -348,6 +346,8 @@ def get_class(name) #:nodoc:
       end
 
       clazz
+    rescue NotFoundError
+      raise NotFoundError, 'class not found'
     end
 
 
@@ -423,8 +423,14 @@ def process_response(response)
 
 
         # return code
-        if 401 == response.code
+        if 400 == response.code
+          raise InvalidRequestError, compose_error_message(response)
+        elsif 401 == response.code
           raise UnauthorizedError, compose_error_message(response)
+        elsif 404 == response.code
+          raise NotFoundError, compose_error_message(response)
+        elsif 409 == response.code
+          raise StateConflictError, compose_error_message(response)
         elsif 500 == response.code
           raise ServerError, compose_error_message(response)
         elsif 2 != (response.code / 100)
@@ -455,6 +461,9 @@ def process_response(response)
       # correspond with expectation.
       def compose_error_message(http_response, max_len=200)
         msg = http_response.body.gsub("\n", ' ')
+        if (matcher = msg.match(/"content": "([^"]+)"/))
+          msg = matcher[1]
+        end
         msg = "#{msg[0..max_len]} ..." if msg.size > max_len
         msg
       end
diff --git a/lib/orientdb4r/rest/restclient_node.rb b/lib/orientdb4r/rest/restclient_node.rb
index 6423cf3..c82ed37 100644
--- a/lib/orientdb4r/rest/restclient_node.rb
+++ b/lib/orientdb4r/rest/restclient_node.rb
@@ -39,7 +39,7 @@ def request(options) #:nodoc:
 
         # store session ID if received to reuse in next request
         sessid = response.cookies[SESSION_COOKIE_NAME]
-        if session_id != sessid and use_session
+        if sessid and session_id != sessid and use_session
           @session_id = sessid
           Orientdb4r::logger.debug "new session id: #{session_id}"
         end
diff --git a/test/test_database.rb b/test/test_database.rb
index 59364a7..de3618a 100644
--- a/test/test_database.rb
+++ b/test/test_database.rb
@@ -80,7 +80,7 @@ def test_create_database
       @client.get_database :database => 'UniT', :user => 'admin', :password => 'admin'
     end
     # creating an existing DB
-    assert_raise Orientdb4r::ServerError do
+    assert_raise Orientdb4r::StateConflictError do
       @client.create_database :database => 'UniT', :user => 'root', :password => DB_ROOT_PASS
     end
     # insufficient rights
diff --git a/test/test_ddo.rb b/test/test_ddo.rb
index da4dcde..9ba107b 100644
--- a/test/test_ddo.rb
+++ b/test/test_ddo.rb
@@ -93,7 +93,7 @@ def test_create_class_extends
     assert_equal 'OUser', clazz.super_class
 
     # bad super class
-    assert_raise Orientdb4r::ServerError do @client.create_class(CLASS, :extends => 'nonExistingSuperClass'); end
+    assert_raise Orientdb4r::InvalidRequestError do @client.create_class(CLASS, :extends => 'nonExistingSuperClass'); end
   end
 
 
diff --git a/test/test_dmo.rb b/test/test_dmo.rb
index bfc9578..58f414e 100644
--- a/test/test_dmo.rb
+++ b/test/test_dmo.rb
@@ -49,7 +49,7 @@ def test_insert
     assert_equal urids.size, @client.query("SELECT FROM #{CLASS} WHERE prop2 = 'linkset'")[0]['friends'].size
 
     # table doesn't exist
-    assert_raise Orientdb4r::ServerError do
+    assert_raise Orientdb4r::InvalidRequestError do
       @client.command "INSERT INTO #{CLASS}x (prop1, prop2, friends) VALUES (1, 'linkset', [#{urids.join(',')}])"
     end
     # bad syntax
@@ -92,7 +92,7 @@ def test_query
     assert_equal 1, gr.select { |e| e if e['@class'] == 'ORole' }.size
 
     # table doesn't exist
-    assert_raise Orientdb4r::ServerError do
+    assert_raise Orientdb4r::InvalidRequestError do
       @client.query 'SELECT FROM OUserX'
     end
     # bad syntax
@@ -105,7 +105,7 @@ def test_query
     assert_instance_of Array, entries
     assert entries.empty?
     # try to find entry in a non-existing cluster
-    assert_raise Orientdb4r::ServerError do @client.query 'SELECT FROM #111:1111'; end
+    assert_raise Orientdb4r::NotFoundError do @client.query 'SELECT FROM #111:1111'; end
     # used for INSERT
     assert_raise Orientdb4r::ServerError do
       @client.query "INSERT INTO #{CLASS} (prop1, prop2, friends) VALUES (0, 'string0', [])"
diff --git a/test/test_document_crud.rb b/test/test_document_crud.rb
index ac5b353..b46f9e8 100644
--- a/test/test_document_crud.rb
+++ b/test/test_document_crud.rb
@@ -157,7 +157,7 @@ def test_batch_bad_params
     doc = @client.create_document( { '@class' => CLASS, 'prop1' => 1, 'prop2' => 'text' })
     assert_raise Orientdb4r::ServerError do @client.batch(nil); end
     assert_raise Orientdb4r::ServerError do @client.batch({:foo => :baz, :bar => :alfa}); end # bad structure
-    assert_raise Orientdb4r::ServerError do @client.batch({:operations => [{:type => :d, :record => {'@rid' => '#123:456'}}]}); end # bad cluster
+    assert_raise Orientdb4r::NotFoundError do @client.batch({:operations => [{:type => :d, :record => {'@rid' => '#123:456'}}]}); end # bad cluster
     assert_nothing_thrown do @client.batch({:operations => [{:type => :d, :record => {'@rid' => "##{doc.doc_rid.cluster_id}:678"}}]}); end # bad RID is not problem
   end