Outlook "not supported calendar message.ics"

Question

I have got a mail from a colleague declining one instance of a recurring meeting I host every Friday. It doesn't tell me in this decline exactly which date it is referring to.

Here is a screenshot of the email that I get from my colleague telling me he is on holiday this week, with the attachment "not supported calendar message.ics"

If I double click the attachment, I get a popup saying "Add this Internet Calendar to Outlook"

If I click Yes, I then get a popup saying "The operation failed".

Could anyone please advise and help me resolve the issue? I don't feel it's appropriate to have to go back to individuals and ask them which meeting occurrence they are declining, particularly when they are very busy and also high up in the business.

Thank you!