What are you doing here? There couldn't possibly a problem, could there? Ok, these things happen and this is as good a place as any to look for the solution. We will troubleshooting GalleryPlus problems & some known issues:

When trying to sort albums, it throws a javascript popup that says "internal server error". It doesn't write anything to the event log. What's causing this issue? I am unable to sort my galleries.

In order to fix the issue, please open up your web.config, find out the element <system.web> and append the following code like that below:

<system.web>
  ...other elements
  <webServices>
    <protocols>
      <add name="HttpGet" />
      <add name="HttpPost" />
    </protocols>
  </webServices>
</system.web>

Note that if you are using DNN5+, you can easily complete this operation with Host/Configure Manager. And another resource link in Stackoverflow - http://stackoverflow.com/questions/657313/request-format-is-unrecognized-for-url-unexpectedly-ending-in