The admin link to add an order is broken! Instead of linking to /index.php?... it links to /administrator/index.php?... relating in a link of <SITE URL>/administrator//administrator/index.php?...
See: https://prnt.sc/ZF3a2JDZAHX-
YES This is the same in EasyStore 1.2.1!
(also, even if I manually edit the URL, the \ before manage order is wrong, it should be a /)