Index: t3lib/extjs/class.t3lib_extjs_extdirectapi.php
===================================================================
--- t3lib/extjs/class.t3lib_extjs_extdirectapi.php	(revision 7796)
+++ t3lib/extjs/class.t3lib_extjs_extdirectapi.php	(working copy)
@@ -64,24 +64,30 @@
 	public function getAPI($ajaxParams, TYPO3AJAX $ajaxObj) {
 		$filterNamespace = t3lib_div::_GET('namespace');
 
-		// look up into the cache
+			// Check GET-parameter no_cache and extCache setting
+		$noCache = t3lib_div::_GET('no_cache') ? TRUE : ($GLOBALS['TYPO3_CONF_VARS']['SYS']['extCache'] === '0');
+
+			// look up into the cache
 		$cacheIdentifier = 'ExtDirectApi';
 		$cacheHash = md5($cacheIdentifier . $filterNamespace . serialize($this->settings));
-		$cacheContent = t3lib_pageSelect::getHash($cacheHash);
+			// with no_cache always generate the javascript content
+		$cacheContent = $noCache ? '' : t3lib_pageSelect::getHash($cacheHash);
 
-		// generate the javascript content if it wasn't found inside the cache and cache it!
+			// generate the javascript content if it wasn't found inside the cache and cache it!
 		if (!$cacheContent) {
 			$javascriptNamespaces = $this->generateAPI($filterNamespace);
-			t3lib_pageSelect::storeHash(
-				$cacheHash,
-				serialize($javascriptNamespaces),
-				$cacheIdentifier
-			);
+			if (!empty($javascriptNamespaces)) {
+				t3lib_pageSelect::storeHash(
+					$cacheHash,
+					serialize($javascriptNamespaces),
+					$cacheIdentifier
+				);
+			}
 		} else {
 			$javascriptNamespaces = unserialize($cacheContent);
 		}
 
-		// return the generated javascript API configuration
+			// return the generated javascript API configuration
 		if (count($javascriptNamespaces)) {
 			$setup = '
 				if (typeof Ext.app.ExtDirectAPI !== "object") {
@@ -103,6 +109,36 @@
 			$ajaxObj->setJavascriptCallbackWrap(
 				$setup . 'Ext.app.ExtDirectAPI = Object.extend(Ext.app.ExtDirectAPI, |);'
 			);
+		} else {
+			if ($filterNamespace) {
+					// namespace error
+				$errorMessage = sprintf($GLOBALS['LANG']->sL('LLL:EXT:lang/locallang_core.xml:ExtDirect.namespaceError'),
+								__CLASS__, $filterNamespace
+				);
+			}
+			else {
+					// no namespace given
+				$errorMessage = sprintf($GLOBALS['LANG']->sL('LLL:EXT:lang/locallang_core.xml:ExtDirect.noNamespace'),
+								__CLASS__
+				);
+			}
+				// make js multiline message
+			$msg = t3lib_div::trimExplode(LF, str_replace('"', '\"', $errorMessage), TRUE);
+			$errorMessage = '';
+			foreach ($msg as $line) {
+				$errorMessage .= '"' . $line . '" + ' . LF;
+			}
+			$errorMessage = substr(trim($errorMessage), 0, -1);
+				//generate the javascript
+			$ajaxObj->setContentFormat('javascript');
+			$ajaxObj->setJavascriptCallbackWrap('
+				errorMessage = ' . $errorMessage . ';
+				if (typeof console == "object") {
+					console.log(errorMessage);
+				} else {
+					alert(errorMessage);
+				}
+			');
 		}
 	}
 
@@ -142,7 +178,7 @@
 					$numberOfParameters = $reflectionMethod->getNumberOfParameters();
 					$docHeader = $reflectionMethod->getDocComment();
 					$formHandler = (strpos($docHeader, '@formHandler') !== FALSE);
-					
+
 					$javascriptNamespaces[$javascriptNamespace]['actions'][$javascriptObjectName][] = array(
 						'name' => $methodName,
 						'len' => $numberOfParameters,
Index: typo3/backend.php
===================================================================
--- typo3/backend.php	(revision 7796)
+++ typo3/backend.php	(working copy)
@@ -247,9 +247,21 @@
 		foreach ($this->jsFiles as $jsFile) {
 			$this->pageRenderer->addJsFile($jsFile);
 		}
-			// we mustn't compress this file
-		$this->pageRenderer->addJsFile('ajax.php?ajaxID=ExtDirect::getAPI&namespace=TYPO3.Backend', NULL, FALSE);
 
+			// Those lines can be removed once we have at least one official ExtDirect router within the backend.
+		$hasExtDirectRouter = FALSE;
+		if (isset($GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['ExtDirect']) && is_array($GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['ExtDirect'])) {
+			foreach ($GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['ExtDirect'] as $key => $value) {
+				if (strpos($key, 'TYPO3.Backend') !== FALSE) {
+					$hasExtDirectRouter = TRUE;
+					break;
+				}
+			}
+		}
+		if ($hasExtDirectRouter) {
+			$this->pageRenderer->addJsFile('ajax.php?ajaxID=ExtDirect::getAPI&namespace=TYPO3.Backend', NULL, FALSE);
+		}
+
 		$this->generateJavascript();
 		$this->pageRenderer->addJsInlineCode('BackendInlineJavascript', $this->js);
 
Index: typo3/sysext/lang/locallang_core.xml
===================================================================
--- typo3/sysext/lang/locallang_core.xml	(revision 7796)
+++ typo3/sysext/lang/locallang_core.xml	(working copy)
@@ -272,6 +272,13 @@
 			<label index="donateWindow.button_donate">Donate now</label>
 			<label index="donateWindow.button_disable">Never show again</label>
 			<label index="donateWindow.button_postpone">Remind me later</label>
+			<label index="ExtDirect.namespaceError">Ext Direct error in &quot;%s&quot; with namespace: &quot;%s&quot;\n
+Try to clear the TYPO3 cache and / or use paramater no_cache=1 as parameter in URL typo3/ajax.php\n\n
+Check also the following points:\n
+- configuration in ext_localconf.php: registration key should be like &quot;TYPO3.MyExtension.Sample&quot;\n
+- URL typo3/ajax.php: namespace parameter should be like: &quot;TYPO3.MyExtension&quot;\n
+- javascript: method\'s name should be like: &quot;TYPO3.MyExtension.Sample.myMethod&quot;\n</label>
+			<label index="ExtDirect.noNamespace">Ext Direct error in &quot;%s&quot;: no namespace has been found.</label>
 		</languageKey>
 	</data>
 </T3locallang>