Here is a workaround. Please click "tools"->"preferences", unselect "Allow Extension Manager to access Internet" option, then try to install it again.
Thanks so much Yuan! It worked!
Thanks for this.
I was having the same "does not contain a valid signature" error generated by Red Giant's Pluraleyes extension and couldn't figure out what the problem was. Disabling the "Allow Extension Manager to access Internet" option solved the issue.
Trick with changing settings may not work with latest version of Extension Manager.
Revert to previous version of EM to install extensions without error.