Thanks for quick reply. However it didn't help. I made some additional tests and here is what I found.
I can create and add new object to Kentico:
BlogCommentInfo b = new BlogCommentInfo { CommentID = 1, ParentCommentID = 1 };
b.Insert();
But then when I'm trying to get objects from this module, I'm getting exception (exception is in second line):
var bc = BlogCommentInfoProvider.GetBlogComments().Where("CommentID=1");
var first = bc.FirstObject;
So it looks that issue is related with ObjectQuery - but how and why?