Wotcher, young Gus!
My take on this is, it's a value that is invalid for one of the valuesets on one of the people group segments. It could be the segment you're trying to change, OR it could be another segment that you're not interested in at all, but nevertheless contains a value that is no longer valid (because someone has disabled one or more hitherto-valid values). Recall that the API will always attempt to validate the current record prior to validating what you are trying to change. The rest of the message (which you have neglected to share with us, you little tease!) should tell you which segment it's complaining about, and possibly the value it doesn't like as well.
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