New Post: Big error 404 file not found

Look for the following in the web.config:

<friendlyUrl defaultProvider="DNNFriendlyUrl">
      <providers>
        <clear />
        <add name="DNNFriendlyUrl" type="DotNetNuke.Services.Url.FriendlyUrl.DNNFriendlyUrlProvider, DotNetNuke.HttpModules" includePageName="true" regexMatch="[^a-zA-Z0-9 _-]" urlFormat="humanfriendly" />
      </providers>
    </friendlyUrl>

where the above reads urlFormat="humanfriendly" your existing entry probably is set to urlFormat="advanced"

Advanced causes several problems including what you have described.