Hi all. Very sorry for the trouble. A new key has been uploaded, but it still errors on my test. apt-key list | grep -iB1 insync shows that the expiration date has changed though. I’ll take a look a this some more.
$ apt-key list | grep -iB1 insync
pub 2048R/ACCAF35C 2012-09-10 [expires: 2020-09-10]
uid Insynchq Inc <services@insynchq.com>
EDIT: It looks like the command I used to import the key was wrong for some reason. Here are the steps to fix this issue then:
$ sudo apt-key remove ACCAF35C
$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys ACCAF35C
$ sudo apt-get update
Let me know if it works for you.