diff --git a/lib/base.php b/lib/base.php
index 7674c16c26b5b..be2457ba9e9e0 100644
--- a/lib/base.php
+++ b/lib/base.php
@@ -412,7 +412,20 @@ public static function initSession() {
 		ini_set('session.cookie_path', $cookie_path);
 
 		// Let the session name be changed in the initSession Hook
-		$sessionName = OC_Util::getInstanceId();
+		try {
+			$sessionName = OC_Util::getInstanceId();
+		} catch (\OC\HintException $e) {
+			$error = [
+				'error' => $e->getMessage(),
+				'hint' => $e->getHint(),
+			];
+
+			http_response_code(503);
+			OC_Util::addStyle('guest');
+			OC_Template::printGuestPage('', 'error', ['errors' => [$error]]);
+
+			exit;
+		}
 
 		try {
 			// Allow session apps to create a custom session object
diff --git a/lib/private/Config.php b/lib/private/Config.php
index f462bebaf5887..5cccdd5558337 100644
--- a/lib/private/Config.php
+++ b/lib/private/Config.php
@@ -237,7 +237,20 @@ private function writeData() {
 		$content .= var_export($this->cache, true);
 		$content .= ";\n";
 
-		touch ($this->configFilePath);
+		// Check if config directory/file is writable and/or create the config file
+		if (
+			(!is_file($this->configFilePath) && !is_writable(dirname($this->configFilePath)))
+			|| !touch($this->configFilePath)
+			|| !is_writable($this->configFilePath)
+		) {
+			// TODO fix this via DI once it is very clear that this doesn't cause side effects due to initialization order
+			// currently this breaks app routes but also could have other side effects especially during setup and exception handling
+			$url = \OC::$server->getURLGenerator()->linkToDocs('admin-dir_permissions');
+			throw new HintException(
+				"Can't write into config directory!",
+				'This can usually be fixed by giving the webserver write access to the config directory. See ' . $url);
+		}
+
 		$filePointer = fopen($this->configFilePath, 'r+');
 
 		// Prevent others not to read the config
diff --git a/tests/lib/ConfigTest.php b/tests/lib/ConfigTest.php
index 2a4c962034079..92753b7f2836e 100644
--- a/tests/lib/ConfigTest.php
+++ b/tests/lib/ConfigTest.php
@@ -160,4 +160,63 @@ public function testConfigMerge() {
 		unlink($additionalConfigPath);
 	}
 
+	public function testExceptionOnConfigDirNotWritable() {
+		$notWritableDir = $this->randomTmpDir.'not_writable/';
+		mkdir($notWritableDir, 0500);
+		if (is_writable($notWritableDir)) {
+			$this->markTestSkipped("Couldn't ensure that the test directory is not writable");
+		}
+
+		$config = new \OC\Config($notWritableDir, 'testconfig.php');
+
+		$this->expectException(\OC\HintException::class);
+
+		$config->setValue('foobar', 'baz');
+	}
+
+	public function testExceptionOnConfigFileNotWritable() {
+		$configFile = $this->randomTmpDir.'not_writable_config.php';
+		touch($configFile);
+		chmod($configFile, 0400);
+		if (is_writable($configFile)) {
+			$this->markTestSkipped("Couldn't ensure that the test file is not writable");
+		}
+
+		$config = new \OC\Config($this->randomTmpDir, 'not_writable_config.php');
+
+		$this->expectException(\OC\HintException::class);
+
+		$config->setValue('foobar', 'baz');
+	}
+
+	public function testNoExceptionOnConfigDirNotWritableButConfigFileWritable() {
+		$notWritableDir = $this->randomTmpDir.'not_writable/';
+		mkdir($notWritableDir, 0700);
+
+		$configFile = $notWritableDir.'writable_config.php';
+		touch($configFile);
+		chmod($configFile, 0600);
+		if (!is_writable($configFile)) {
+			$this->markTestSkipped("Couldn't ensure that the test file is writable");
+		}
+
+		chmod($notWritableDir, 0500);
+		if (is_writable($notWritableDir)) {
+			$this->markTestSkipped("Couldn't ensure that the test directory is not writable");
+		}
+
+		$config = new \OC\Config($notWritableDir, 'writable_config.php');
+		$config->setValue('foobar', 'baz');
+
+		$this->assertTrue(true, 'No exception when writing to config file');
+
+		chmod($notWritableDir, 0700);
+	}
+
+	public function testNoExceptionOnCreatingConfigFileInWritableConfigDir() {
+		$config = new \OC\Config($this->randomTmpDir, 'this_file_does_not_exist.config.php');
+		$config->setValue('foobar', 'baz');
+
+		$this->assertTrue(true, 'No exception when creating config file');
+	}
 }