Workaround:

#set($globalHelper=$action.getGlobalHelper())
#set($renderedContent=$globalHelper.renderConfluenceMacro($body))
${renderedContent}

http://confluence.atlassian.com/display/DOC/User+Macros?focusedCommentId=134296#comment-134296

Hm. Seems as if this only works for simple bodies. I. e. you either have to use the first workaround and ignore the errors in the Global Templates Preview or you get even more complex. The following is an example-macro which just renders the body (but of course you might extend $wiki as you like):

#set($wiki=$body)
#set($globalHelper=$action.getGlobalHelper())
#if($content) ## i. e. we render a normal page
#set($renderer=$globalHelper.getWikiStyleRenderer())
#set($context=$content.toPageContext())
#set($xhtml=$renderer.convertWikiToXHtml($context, $wiki))
#else ## we are e. g. in Global Template Preview
#set($xhtml=$globalHelper.renderConfluenceMacro($wiki))
#end
${xhtml}
-------------------------
I personally don't understand why renderConfluenceMacro and convertWikiToXHtml differ that much in their behavior. I cannot state exactly what makes renderConfluenceMacro different but it seems nested calls of user-macros and then using the link-to-Macro breaks because the link-to-macro does not get a context (error message: link-to: You must supply a space key for the 'news' target in this context.)

Below are the nested macros I use. Rendering {test-macro} on a page will break with in Version 2

test-macro
---------------
#set($wiki="{cm-newsticker}")
#set($globalHelper=$action.getGlobalHelper())
#set($renderer=$globalHelper.getWikiStyleRenderer())
#set($context=$content.toPageContext())
#set($xhtml1=$renderer.convertWikiToXHtml($context, $wiki))
#set($xhtml2=$globalHelper.renderConfluenceMacro($wiki))
<dl>
<dt>Version 1</dt>
<dd>${xhtml1}</dd>
<dt>Version 2</dt>
<dd>${xhtml2}</dd>
</dl>
---------------

cm-newsticker
--------------------
#set($globalHelper=$action.getGlobalHelper())
#set($wiki="{cm-blue-panel:Newsticker}")
#set($wiki="${wiki}{blog-posts:3|content=titles}")
#set($wiki="${wiki} {link-to:news}more{link-to}")
#set($wiki="${wiki}{cm-blue-panel}")
#if($content)
#set($renderer=$globalHelper.getWikiStyleRenderer())
#set($context=$content.toPageContext())
#set($xhtml=$renderer.convertWikiToXHtml($context, $wiki))
#else
#set($xhtml=$globalHelper.renderConfluenceMacro($wiki))
#end
${xhtml}
----------------------

cm-blue-panel
---------------------------

1. 1. Usage:
      ##
   2. {cm-blue-panel:<title>}<content>{cm-blue-panel}

      ##

   3. Arguments
      ##
      #set($title=$param0)
      ##
   4. Constants
      ##
      #set($titleBackground="#5296D6")
      #set($contentBackground="#ADCBEF")
      ##
   5. Panel-Configuration
      ##
      #set($panelStart="{panel:title=${title}|borderStyle=none|bgColor=${contentBackground}|titleBGColor=${titleBackground}}")
      #set($panelEnd="
      ")
      #set($wiki="${panelStart}${body}${panelEnd}")
      ##
   6. Renderer-Preparations
      ##
      #set($globalHelper=$action.getGlobalHelper())
      #if($content)
      #set($renderer=$globalHelper.getWikiStyleRenderer())
      #set($context=$content.toPageContext())
      #set($xhtml=$renderer.convertWikiToXHtml($context, $wiki))
      #else
      #set($xhtml=$globalHelper.renderConfluenceMacro($wiki))
      #end
      ${xhtml}
      -------------------------------