Yea, I got 3 sites with the exact same issue, all of which have to use plain editor for now, i'm going through source code trying to figure out what the issue is, but can't seem to figure out how to debug it just yet... In my case i HAVE to use telerik editor...

Same problem.
    DNN v5.5.1
    Blog v3.5 & v4.0.0
    .NET 4.0.30319

With Blog v.3.5, when I saved the blog entry, the content in the editor panes would disappear and I was getting an "Entry is Required" message...so I upgraded to Blog v4.0 to see if that fixed the problem.  Now, when I try to add a new blog entry with Blog v4.0 using Telerik editor & Save as Draft, I get the following UI error & the event at the bottom of this post:
    A critical error has occurred. 
    Object reference not set to an instance of an object.

If I change the blog editor to Basic Text Box or to the FCK html editor, I am able to save the blog entry successfully.  I am also able to create basic HTML content on other pages using the Telerik editor.  It's like the content of the Telerik editor is being lost when trying to save a blog entry.

I think this may be related to .NET 4.0 because I was able to use Telerik with the blog module previously with .NET 2.0.50727.

AssemblyVersion: 5.5.1
PortalID: 2
PortalName: Knowledgebase
UserID: x
UserName: xxxxxxxxx
ActiveTabID: 268
ActiveTabName: Sandbox
RawURL: /emr/Resources/Sandbox/tabid/268/ctl/Edit_Entry/mid/802/Default.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://intranettest/emr/Resources/Sandbox/tabid/268/ctl/Edit_Entry/mid/802/Default.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.2; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; MS-RTC LM 8)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: bfe4f608-6282-436b-bc51-d1e272d1dc23
InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.Modules.Blog.EditEntry.cmdDraft_Click
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Modules.Blog.EditEntry.cmdDraft_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e) at System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument) at System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---
Source:
Server Name: INTRANETTEST

I have the same problem

AssemblyVersion: 5.5.1
PortalID: 6
PortalName: Dauntless Gaming
UserID: 1
UserName: Dauntless
ActiveTabID: 155
ActiveTabName: Quarantine Zone
RawURL: /Articles/QuarantineZone/tabid/155/ctl/Edit_Entry/mid/609/BlogID/6/Default.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://www.dauntlessgaming.com/Articles/QuarantineZone/tabid/155/ctl/Edit_Entry/mid/609/BlogID/6/Default.aspx
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.10) Gecko/20100914 Firefox/3.6.10
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 4532b77c-16be-4cc9-911c-da19be74d504
InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.Modules.Blog.EditEntry.cmdDraft_Click
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Modules.Blog.EditEntry.cmdDraft_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---
Source:
Server Name: WEB3