Have a look at http://silverstripe.org/blog-module-forum/show/265986 and the ticket - http://open.silverstripe.org/ticket/4376. I think you should be able to fix this by updating to a daily build of blog.

Ahhh wait!! I just clicked on the "subscribe via rss" module link and got the following error:

"Action 'rss' isn't allowed on class BlogHolder_Controller"

...and just now I went to my /admin page to check some content in CMS and I got this enormous error....

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[User Error] DataObject::getComponent(): Unknown 1-to-1 component 'SideBar' on class 'BlogHolder'
GET /admin

Line 1059 in /home/mrwindup/www/www/sapphire/core/model/DataObject.php
Source

1050
1051          // If no component exists, create placeholder object
1052          if(!$component) {
1053             $component = $this->createComponent($componentName);
1054          }
1055
1056          $this->components[$componentName] = $component;
1057          return $component;
1058       } else {
1059          user_error("DataObject::getComponent(): Unknown 1-to-1 component '$componentName' on class '$this->class'", E_USER_ERROR);
1060       }
1061    }
1062
1063    /**
1064     * A cache used by component getting classes
1065     * @var array

Trace

* DataObject::getComponent(): Unknown 1-to-1 component 'SideBar' on class 'BlogHolder'
Line 1059 of DataObject.php
* DataObject->getComponent(SideBar)
Line 39 of WidgetAreaEditor.php
* WidgetAreaEditor->Value()
* call_user_func_array(Array,Array)
Line 408 of ViewableData.php
* ViewableData->XML_val(Value,,1)
Line 19 of .cache.home.mrwindup.www.www.cms.templates.WidgetAreaEditor.ss
* include(/home/mrwindup/www/www/silverstripe-cache/.cache.home.mrwindup.www.www.cms.templates.WidgetAreaEditor.ss)
Line 354 of SSViewer.php
* SSViewer->process(WidgetAreaEditor)
Line 773 of ViewableData.php
* ViewableData->renderWith(WidgetAreaEditor)
Line 12 of WidgetAreaEditor.php
* WidgetAreaEditor->FieldHolder()
* call_user_func_array(Array,Array)
Line 408 of ViewableData.php
* ViewableData->XML_val(FieldHolder,,1)
Line 74 of .cache.home.mrwindup.www.www.sapphire.templates.TabSetFieldHolder.ss
* include(/home/mrwindup/www/www/silverstripe-cache/.cache.home.mrwindup.www.www.sapphire.templates.TabSetFieldHolder.ss)
Line 354 of SSViewer.php
* SSViewer->process(TabSet)
Line 773 of ViewableData.php
* ViewableData->renderWith(TabSetFieldHolder)
Line 62 of TabSet.php
* TabSet->FieldHolder()
* call_user_func_array(Array,Array)
Line 408 of ViewableData.php
* ViewableData->XML_val(FieldHolder,,1)
Line 59 of .cache.home.mrwindup.www.www.sapphire.templates.TabSetFieldHolder.ss
* include(/home/mrwindup/www/www/silverstripe-cache/.cache.home.mrwindup.www.www.sapphire.templates.TabSetFieldHolder.ss)
Line 354 of SSViewer.php
* SSViewer->process(TabSet)
Line 773 of ViewableData.php
* ViewableData->renderWith(TabSetFieldHolder)
Line 62 of TabSet.php
* TabSet->FieldHolder()
* call_user_func_array(Array,Array)
Line 408 of ViewableData.php
* ViewableData->XML_val(FieldHolder,,1)
Line 70 of .cache.home.mrwindup.www.www.sapphire.templates.Includes.Form.ss
* include(/home/mrwindup/www/www/silverstripe-cache/.cache.home.mrwindup.www.www.sapphire.templates.Includes.Form.ss)
Line 354 of SSViewer.php
* SSViewer->process(Form)
Line 773 of ViewableData.php
* ViewableData->renderWith(Array)
Line 982 of Form.php
* Form->forTemplate()
Line 424 of ViewableData.php
* ViewableData->XML_val(EditForm,,1)
Line 14 of .cache.home.mrwindup.www.www.cms.templates.Includes.CMSMain_right.ss
* include(/home/mrwindup/www/www/silverstripe-cache/.cache.home.mrwindup.www.www.cms.templates.Includes.CMSMain_right.ss)
Line 354 of SSViewer.php
* SSViewer->process(CMSMain)
Line 773 of ViewableData.php
* ViewableData->renderWith(Array)
Line 484 of LeftAndMain.php
* LeftAndMain->Right()
* call_user_func_array(Array,Array)
Line 408 of ViewableData.php
* ViewableData->XML_val(Right,,1)
Line 71 of .cache.home.mrwindup.www.www.cms.templates.LeftAndMain.ss
* include(/home/mrwindup/www/www/silverstripe-cache/.cache.home.mrwindup.www.www.cms.templates.LeftAndMain.ss)
Line 354 of SSViewer.php
* SSViewer->process(CMSMain)
Line 175 of Controller.php
* Controller->handleAction(HTTPRequest)
Line 129 of RequestHandler.php
* RequestHandler->handleRequest(HTTPRequest)
Line 122 of Controller.php
* Controller->handleRequest(HTTPRequest)
Line 277 of Director.php
* Director::handleRequest(HTTPRequest,Session)
Line 121 of Director.php
* Director::direct(/admin)
Line 118 of main.php

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...I am a bit surprised that just updating the blog module could cause such damage to the whole of CMS... should I revert back to my old blog module or is there another workaround for this?

Thanks for ALL your help!