I restart the php on the server, and now can access site again. But problem with course categories still active.
The Joomla Debug says that:
0 Class 'StringHelper' not found
/home/uf6x56hf/domains/learnsafety.it/public_html/administrator/components/com_splms/models/coursescategory.php:96
Call stack
Function Location
1 () JROOT/administrator/components/com_splms/models/coursescategory.php:96
2 SplmsModelCoursescategory->save() JROOT/libraries/src/MVC/Controller/FormController.php:775
3 Joomla\CMS\MVC\Controller\FormController->save() JROOT/libraries/src/MVC/Controller/BaseController.php:710
4 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/administrator/components/com_splms/splms.php:24
5 require_once() JROOT/libraries/src/Component/ComponentHelper.php:402
6 Joomla\CMS\Component\ComponentHelper::executeComponent() JROOT/libraries/src/Component/ComponentHelper.php:377
7 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/AdministratorApplication.php:101
8 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT/libraries/src/Application/AdministratorApplication.php:159
9 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:196
10 Joomla\CMS\Application\CMSApplication->execute() JROOT/administrator/index.php:51
Please help soon