Hi Ulu,
Was this a clean checkout or an svn update?
In any case you can fix it by copying the Brettle.Web.NeatHtml.dll from the _libs folder into Web/bin
I'm thinking you have an older version lodged there that didn't get updated during the build for some reason. Only other possible cause is if you don't have the newest Web.config you might be missing the control declaration for NeatHtml.UntrustedContent in the Pages Controls section of Web.config
Hope it helps,
Joe
ps, you project looks interesting!