From my experience, this is happens occasionally and is not limited to IHG.
Some websides block certain VPN IP adresses, because these are also used by robots or spammers and this actually is also a way to protect them from attacks.
I was able to access IHG using a VPN sometimes and sometimes I am not. This also applies for Starwood, Lufthansa, Swiss and some others. Blacklane and Expedia use a captcha in this cases and so does Lufthansa and Swiss since a couple of months now.

Planespotters.net for example tells you, if they block only these IP adress or the VPN at all. Sometimes it helps switching to another server.

This "problem" is in my eyes not limited to IHG but depending on the VPN service you use. It sometimes help to clear you cache and cookies of your browers from previous setting before starting the VPN and accessing the side.