The reason for the error is because if PI Buffer Subsystem is used to buffer and fan data to a PI Collective, it locks up the snapshots of the points that PI Buffer Subsystem is handling. I think this Techsupport Article about this error will be helpful for you if you want to unlock the points temporarily (see Solution 3, Method 1)
Taking a step away from your question. I can't help but to notice that you are using PISDK.IPICollective interface when testing if the PI Server is a collective and also getting a list of members in the collective. I am guessing that one of the reason is that you are building on top of an existing app that uses PI SDK. Instead of creating your own list of OSIsoft.AF.PI.PIServer to reference the collective members, you can also use the OSIsoft.AF.PI.PICollective object where the list of collective members will be accessible.
Two things to note: AF SDK 2.5 does not yet support PIBufss. AF SDK 2.6 is supposed to. Even then there are certain modes not supported by PIBuffss, they are around removing/replacing values on a PI Collective.