DNN Blog does not allow to save new posts or comments

We upgraded DNN from 6 to 9.1.1. The Blog module was upgraded to 6.00.5, then to 6.3.0, then to 6.04.01. Now the blog shows the posts and comments, but does not allow to create new posts or comments.

Error message:

ModuleId:454
ModuleDefId:123
FriendlyName:
ModuleControlSource:DesktopModules/Blog/PostEdit.ascx
AbsoluteURL:/Default.aspx
DefaultDataProvider:DotNetNuke.Data.SqlDataProvider, DotNetNuke
ExceptionGUID:db57f36e-cd6f-4541-a389-dc26f122f9e6
AssemblyVersion:9.1.1
PortalId:0
UserId:24
TabId:88
RawUrl:/El-Blog/ctl/PostEdit/mid/454/Post/323
Referrer:************El-Blog/ctl/PostEdit/mid/454/Post/323
UserAgent:Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0
ExceptionHash:EpWiJC2IEM7+2eVutsX4Ugr8ANo=
Message:Object reference not set to an instance of an object.
StackTrace:
InnerMessage:Object reference not set to an instance of an object.
InnerStackTrace:
at DotNetNuke.Modules.Blog.Integration.NotificationController.PostPendingApproval(BlogInfo objBlog, PostInfo objPost, Int32 portalId, String summary, String title) at DotNetNuke.Modules.Blog.PostEdit.cmdSave_Click(Object sender, EventArgs e)