I would suggest to use XOOPS 2.5.11 and min PHP 7.4
latest version of 2.5.11 you can find here:https://github.com/XOOPS/XoopsCore25
I made a lot of improvement in modulebuilder but I didn't test it with 2.5.10
maybe this can solve the problems
// Get Theme Form
\xoops_load('XoopsFormLoader');
$form = new \XoopsThemeForm('My form for testing cascading select', 'formTest', $_SERVER['REQUEST_URI'], 'post', true);
$form->setExtra('enctype="multipart/form-data"');
$myExampleTray1 = new XoopsFormElementTray('Example Tray 1');
$mySelect1 = new XoopsModules\Wgsimpleacc\Form\FormSelectCascading('Caption Select 1', 'select1', '2', 15);
$mySelect1->setType(1);
$arrSelect1 = [
['id' => '1', 'text'=>'Sourceelement 1', 'rel'=> '0', 'init'=> '0'],
['id' => '2', 'text'=>'Sourceelement 2', 'rel'=> '0', 'init'=> '0'],
['id' => '3', 'text'=>'Sourceelement 3', 'rel'=> '0', 'init'=> '0'],
];
$mySelect1->setCustomOptions($arrSelect1);
$myExampleTray1->addElement($mySelect1);
$mySelect2 = new XoopsModules\Wgsimpleacc\Form\FormSelectCascading('Caption Select 2', 'select2', '4', 15);
$mySelect2->setType(2);
$arrSelect2 = [
['id' => '1', 'text'=>'Targetelement 1, linked to Sourceelement 1', 'rel'=> '1', 'init'=> '2'],
['id' => '1', 'text'=>'Targetelement 1, linked to Sourceelement 2', 'rel'=> '2', 'init'=> '2'],
['id' => '1', 'text'=>'Targetelement 1, linked to Sourceelement 3', 'rel'=> '3', 'init'=> '2'],
['id' => '2', 'text'=>'Targetelement 2, linked to Sourceelement 1', 'rel'=> '1', 'init'=> '2'],
['id' => '3', 'text'=>'Targetelement 3, linked to Sourceelement 1', 'rel'=> '1', 'init'=> '2'],
['id' => '3', 'text'=>'Targetelement 3, linked to Sourceelement 3', 'rel'=> '3', 'init'=> '2'],
['id' => '4', 'text'=>'Targetelement 4, linked to Sourceelement 2', 'rel'=> '2', 'init'=> '2'],
['id' => '5', 'text'=>'Targetelement 5, linked to Sourceelement 2', 'rel'=> '2', 'init'=> '2'],
];
$mySelect2->setCustomOptions($arrSelect2);
$myExampleTray1->addElement($mySelect2);
$form->addElement($myExampleTray1);
$form->addElement(new \XoopsFormHidden('op', 'save'));
$form->addElement(new \XoopsFormButtonTray('', _SUBMIT, 'submit', '', false));
$GLOBALS['xoopsTpl']->assign('form', $form->render());
$xoopsMailer = xoops_getMailer();
$xoopsMailer->multimailer->isSMTP();
$xoopsMailer->multimailer->Port = $my_port_out; // set the SMTP port
$xoopsMailer->multimailer->Host = $my_server_out; //sometimes necessary to repeat
$xoopsMailer->multimailer->SMTPAuth = true;
$xoopsMailer->multimailer->SMTPSecure = $my_securetype_out;
$xoopsMailer->multimailer->Username = $my_username; // SMTP account username
$xoopsMailer->multimailer->Password = $my_password; // SMTP account password
$xoopsMailer->multimailer->SMTPDebug = 0;
...