Categories cannot be saved

[Karl1]

When I open an existing category (and change something or not) it cannot be saved.
error message:
An error has occurred.

0 Call to a member function format() on bool

Call Stack
# Function Location
1 () JROOT/libraries/src/Form/Field/CalendarField.php:400
2 Joomla\CMS\Form\Field\CalendarField->filter() JROOT/libraries/src/Form/Form.php:1047
3 Joomla\CMS\Form\Form->filter() JROOT/libraries/src/Form/Form.php:991
4 Joomla\CMS\Form\Form->process() JROOT/libraries/src/MVC/Model/FormModel.php:219
5 Joomla\CMS\MVC\Model\FormModel->validate() JROOT/administrator/components/com_phocagallery/controllers/phocagalleryc.php:238
6 PhocaGalleryCpControllerPhocaGalleryc->save() JROOT/libraries/src/MVC/Controller/BaseController.php:730
7 Joomla\CMS\MVC\Controller\BaseController->execute() JROOT/administrator/components/com_phocagallery/phocagallery.php:50
8 require_once() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:71
9 Joomla\CMS\Dispatcher\LegacyComponentDispatcher::Joomla\CMS\Dispatcher\{closure}() JROOT/libraries/src/Dispatcher/LegacyComponentDispatcher.php:73
10 Joomla\CMS\Dispatcher\LegacyComponentDispatcher->dispatch() JROOT/libraries/src/Component/ComponentHelper.php:361
11 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/AdministratorApplication.php:150
12 Joomla\CMS\Application\AdministratorApplication->dispatch() JROOT/libraries/src/Application/AdministratorApplication.php:195
13 Joomla\CMS\Application\AdministratorApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:306
14 Joomla\CMS\Application\CMSApplication->execute() JROOT/administrator/includes/app.php:58
15 require_once() JROOT/administrator/index.php:32

[Jan]

Hi, see:
https://www.phoca.cz/forum/viewtopic.php?t=65575

Jan