I see in apache error log that there is an error: Uncaught TypeError: Unsupported operand types: string + int in line 2515 from functions.php
Line 2515 reads: $id_mod = $id + 1;
With replacing that line and by casting the $id variable to an integer before performing the addition the error seems solved.
So in functions.php change:
if($id !== '' && $id !== null) {
$id_mod = $id + 1;
} else {
$id_mod = 0;
}
TO:
if ($id !== '' && $id !== null) {
$id_mod = (int)$id + 1;
} else {
$id_mod = 0;
}