public partial class ImageConversions : System.Web.UI.Page
protected void Page_Load(object sender, EventArgs e)
clsAction action = new clsAction();
That's a generic message, saying something went wrong during execution of the code on the server. There'll be an exception probably, but that should have shown in your text-file now. Unless, the user isn't allowed to write there. Perhaps it's easier to display the message using Response.Write. I'm not an ASP-programmer, but the code "looks" like it should run.
I'm guessing that something goes wrong either by decoding the picture from Base64 (try and save the raw data, might be a nice clue for debugging) or the users' rights. Easy way to check the latter is by creating a small test-page that writes a line of text to that location.