New Product, Advanced Options, Finish not working

I have installed PowerStore3 on 1and1.co.uk. It is the basic vanilla product. I have not changed anything. I am testing the functions. Adding Categories works fine.

Next I am trying to Add Products. I am completing the basic menu with valid information, but not ticking the "make available online" box (another thread suggested that box as a problem). I am ticking the "Advanced options" box, to add sizes. When I have completed the sizes, I press Finish. I get taken to Products_Update.php?ProductID=22 (productid changes as I test the add function) and presented with a blank screen.

I expected to be either taken back to a blank Add Product screen, or given a confirmation screen, but even though I have tried more than once to add just one single product, I keep getting this screen.

However, when I relogin to PowerStore3 Admin (going back gives me an error, "Web page has expired"), the product is there ~ I can successfully search for it.

I am using IE8 and Firefox 3.5.5 to access the store.

Can you tell me what I should see after I press the Finish button on the Advanced Options screen?
Can you suggest any reason why I am getting a blank screen?

Regards, Nick

I went on to test other functions on my 1and1.co.uk installation. "Users" from Admin, and all functions under it, take me to a 404 error msg. This feels very strange. As I said, it is a simple vanilla installation, the only addition being the .htaccess file saying "AddType x-mapp-php5 .php" in the root folder.

Further information:
Both the Product Add and Users function works perfectly well on my Apache / php / mySQL server at home. The joy here is that I now know I have not done anything wrong with the setup / installation.

Both times I have uploaded PS3 from Dreamweaver CS3. For the home server I used webdav, and for 1and1.co.uk I used FTP. At the moment, I am just testing, so it is very much the default code, the default settings. My site on 1and1.co.uk is index.php if that helps.