Thanks for the update.
With the following workaround I was able to eliminate the errors. Up to now there are no side effects.
lib/Froxlor/PhpHelper.php:67
// $subject = htmlentities($subject, $quote_style, $charset);
$subject = htmlentities((string)$subject, $quote_style, $charset);
vendor/froxlor/idna-convert-legacy/src/IdnaConvert.php:191
// $input = trim($input);
$input = trim((string)$input);
admin_domains.php:688
// $row['termination_date'] = str_replace("0000-00-00", "", $row['termination_date']);
$row['termination_date'] = str_replace("0000-00-00", "", (string)$row['termination_date']);
customer_domains.php:509
// $row['termination_date'] = str_replace("0000-00-00", "", ($row['termination_date']);
$row['termination_date'] = str_replace("0000-00-00", "", (string)$row['termination_date']);