You should remove that handler mapping declaration, if you used the Web.config file that shipped with version 2.3.7.6 you would not get this error because it does not have that handler mapping. It does not need a handler mapping because the file exists on disk. In older versions it was needed but we completely replaced the metaweblog support with a new implementation in version 2.3.7.5
You should always use the web.config file that ships with mojoPortal (instead of keeping your old one) and re-apply any modifications such as machine key.
Hope that helps,
Joe