I encountered this error when one of the properties in Hubspot was some kind of enumerable (checkboxes for examples). Those properties requires a set of predefined values to be defined. If the value you send from Segment is not in this predefined set, you get this error.
I never tried the group call though, so it might be somethings completely different.
Normally, there is a panel in Segment where you can get more details on the error I think (get the actuel response payload of Hubspot). It might be helpful to get the actual reason of the error.