I have the same problem and YES I see certificate error.
What can I do to solve the problem? :(
[root@plesk ~]# wget Plesk Extensions
--2022-04-28 20:25:09-- Plesk Extensions
Resolving ext.plesk.com... 212.102.55.131, 212.102.55.139, 2a02:6ea0:d500::5, ...
Connecting to...