Thank you for your replies.

I discovered that the problem with show.aspx was that I was not providing an ?album argument.

The problem with my pages was... well, I feel a bit stupid, but nevertheless:

I write all my code by hand (for various reasons), using a non-MS IDE. I had forgotten to insert the <form runat="server" ID="mainform"> tags into the source of my page.

It works now (well, more or less).

Thanks for your help!