diff --git a/actions/admin/settings/122.froxlorvhost.php b/actions/admin/settings/122.froxlorvhost.php
index fa2ea7c5..bd682279 100644
--- a/actions/admin/settings/122.froxlorvhost.php
+++ b/actions/admin/settings/122.froxlorvhost.php
@@ -177,6 +177,10 @@ return [
 					'type' => 'text',
 					'default' => 'froxlorlocal',
 					'string_emptyallowed' => false,
+					'plausibility_check_method' => [
+						'\\Froxlor\\Validate\\Check',
+						'checkSystemUsername'
+					],
 					'save_method' => 'storeSettingWebserverFcgidFpmUser',
 					'websrv_avail' => [
 						'apache2'
@@ -246,6 +250,10 @@ return [
 					'type' => 'text',
 					'default' => 'froxlorlocal',
 					'string_emptyallowed' => false,
+					'plausibility_check_method' => [
+						'\\Froxlor\\Validate\\Check',
+						'checkSystemUsername'
+					],
 					'save_method' => 'storeSettingWebserverFcgidFpmUser',
 					'visible' => Settings::Get('phpfpm.enabled') && call_user_func([
 						'\Froxlor\Settings\FroxlorVhostSettings',
diff --git a/actions/admin/settings/130.webserver.php b/actions/admin/settings/130.webserver.php
index fd149401..dbe6d527 100644
--- a/actions/admin/settings/130.webserver.php
+++ b/actions/admin/settings/130.webserver.php
@@ -104,6 +104,10 @@ return [
 					'varname' => 'httpuser',
 					'type' => 'text',
 					'default' => 'www-data',
+					'plausibility_check_method' => [
+						'\\Froxlor\\Validate\\Check',
+						'checkSystemUsername'
+					],
 					'save_method' => 'storeSettingWebserverFcgidFpmUser'
 				],
 				'system_httpgroup' => [
diff --git a/lib/Froxlor/Validate/Check.php b/lib/Froxlor/Validate/Check.php
index 70c3c9f5..bf04379f 100644
--- a/lib/Froxlor/Validate/Check.php
+++ b/lib/Froxlor/Validate/Check.php
@@ -359,4 +359,41 @@ class Check
 
 		return [self::FORMFIELDS_PLAUSIBILITY_CHECK_OK];
 	}
+
+	/**
+	 * @param $fieldname
+	 * @param $fielddata
+	 * @param $newfieldvalue
+	 * @param $allnewfieldvalues
+	 * @return array|int[]
+	 * @throws \Exception
+	 */
+	public static function checkSystemUsername($fieldname, $fielddata, $newfieldvalue, $allnewfieldvalues)
+	{
+		if (empty($newfieldvalue) || $fielddata['value'] == $newfieldvalue) {
+			$returnvalue = [
+				self::FORMFIELDS_PLAUSIBILITY_CHECK_OK
+			];
+		} elseif (function_exists('posix_getpwnam') && posix_getpwnam($newfieldvalue) == false) {
+			$returnvalue = [
+				self::FORMFIELDS_PLAUSIBILITY_CHECK_ERROR,
+				'local_user_invalid'
+			];
+		} else {
+			// user exists, but cannot be one of the froxlor-customers
+			$sel_stmt = Database::prepare("SELECT COUNT(*) as numUsers FROM `" . TABLE_PANEL_CUSTOMERS . "` WHERE `loginname` = :username");
+			$result = Database::pexecute_first($sel_stmt, [':username' => $newfieldvalue]);
+			if ($result && $result['numUsers'] > 0) {
+				$returnvalue = [
+					self::FORMFIELDS_PLAUSIBILITY_CHECK_ERROR,
+					'local_user_isfroxloruser'
+				];
+			} else {
+				$returnvalue = [
+					self::FORMFIELDS_PLAUSIBILITY_CHECK_OK
+				];
+			}
+		}
+		return $returnvalue;
+	}
 }
diff --git a/lng/de.lng.php b/lng/de.lng.php
index b24e1270..bc7c66a0 100644
--- a/lng/de.lng.php
+++ b/lng/de.lng.php
@@ -966,6 +966,8 @@ return [
 		'no_wwwcnamae_ifwwwalias' => 'Es kann kein CNAME Eintrag für "www" angelegt werden, da die Domain einen www-Alias aktiviert hat. Ändere diese Einstellung auf "Kein Alias" oder "Wildcard Alias"',
 		'local_group_exists' => 'Die angegebene Gruppe existiert bereits auf dem System',
 		'local_group_invalid' => 'Der angegebene Gruppen-Name ist nicht gültig',
+		'local_user_invalid' => 'Der angegebene Benutzer-Name ist nicht gültig oder existiert nicht',
+		'local_user_isfroxloruser' => 'Der angegebene Benutzer-Name ist ein von froxlor verwalteter Benutzer und kann in diesem Kontext nicht verwendet werden.',
 		'invaliddnsforletsencrypt' => 'Die DNS-Einträge der Domain enthalten keine der gewählten IP Adressen. Let\'s Encrypt Zertifikats-Erstellung ist nicht möglich.',
 		'notallowedphpconfigused' => 'Nutzung einer PHP-Konfiguration welche nicht dem Kunden zugeordnet ist',
 		'pathmustberelative' => 'Der Benutzer hat nicht die benötigten Berechtigungen, um Pfade außerhalb des Kunden-Heimatverzeichnisses anzugeben. Bitte einen relativen Pfad angeben (kein führendes /).',
diff --git a/lng/en.lng.php b/lng/en.lng.php
index e79abef9..97239bc7 100644
--- a/lng/en.lng.php
+++ b/lng/en.lng.php
@@ -1039,6 +1039,8 @@ return [
 		'no_wwwcnamae_ifwwwalias' => 'Cannot set CNAME record for "www" as domain is set to generate a www-alias. Please change settings to either "No alias" or "Wildcard alias"',
 		'local_group_exists' => 'The given group already exists on the system.',
 		'local_group_invalid' => 'The given group name is invalid',
+		'local_user_invalid' => 'The given user name is invalid or does not exist',
+		'local_user_isfroxloruser' => 'The given user name is a froxlor managed username and cannot be used in this context',
 		'invaliddnsforletsencrypt' => 'The domains DNS does not include any of the chosen IP addresses. Let\'s Encrypt certificate generation not possible.',
 		'notallowedphpconfigused' => 'Trying to use php-config which is not assigned to customer',
 		'pathmustberelative' => 'The user does not have the permission to specify directories outside the customers home-directory. Please specify a relative path (no leading /).',