Updated to CS2.1 SP1 and fixed an issue
I just upgraded my site to Community Server 2.1 SP1. I wouldn't post this if I did not encounter an issue, though you may experience this error only if you use my coComment support for CS. In fact, it occurs only if you use BlogThreadQuery and set ReturnFullThread to false. In this case, the stored procedure cs_weblog_PostSet does not return ApplicationPostType for found posts, but the code tries to retrieve it from the IDataReader.
I attached a corrected version of that SP to this post. In fact, I only added P.ApplicationPostType, in line 128.