Cannot Add new payment method

-- url of the page with the problem -- : www.lefko-shop.gr
-- HikaShop version -- : 4.4.4
-- Joomla version -- : 3.9.10
-- PHP version -- : 7.4

Hello,
I cannot add new payment plugin.
When I try to click the +New button it does not redirect to choose one of the plugins and it shows the hikashop menu without css... like following

It was working fine in the past...

Hi,

Please activate the "error reporting" and "debug system" settings of the Joomla configuration and try again.
It will display an error message at the end of that page which will help us understand what's going on since we don't have the issue on our end.