Unexpected 'Cannot Verify Server Identity' alert on iPad Air

Since yesterday I have had a popup/alert dialog appear on my iPad a few times. It says:

‘Cannot Verify Server Identity. The identity of "subscriptions.thismonkey.com" cannot be verified.

When I click the Details option it displays the following ‘Certificate’ information:

thismonkey.com. Issued by R10. Not Trusted. Expired 16/09/2025, 14:02:59

What is it? Should I be concerned?

[Edited by Moderator]

Original Title: Unexpected alert/popup?