Any chance you are looking at a different record--one without a valid URL reference for the viewer when you are in preview mode?
When you enter preview mode, FileMaker assumes you want to see what you will get if you were to print the entire found set of records with the "records being browsed" print option and previews the first page of such a report. If you are currently on a record other than the first record in the found set, this preview may thus show data from a completely different record than you had up in Browse mode. In such cases, you can click on the "book" control while in preview mode to move through the previewed pages until you get to the same record that was current in Browse mode, or you can manipulate your found set to isolate your current record in a single record found set before previewing.
During this saga I've discovered what you write about when going to Preview mode and seeing the first record. Unfortunately that is not the problem. In the test.fp7 all I have is one record with one layout with one webviewer.Of course it works fine on my Mac.
I've removed all my plugins but that didn't help. I assume since I can't find any similar post about this issue that it is specific to my PC. I've updated it with the latest Win 7 updates. Preview used to work correctly on this PC. I guess I might go to a previous restore point but I'm not optimistic about that.
Interesting result, I find this is not an issue with Windows XP.
Let's try a simple test, create a brand new simple file with a web viewer and see if it shows the same problem. If it does, go to Report an Issue and file a bug report.
I did create a simple test.fp7 with just the one record, one layout, one webviewer. Works on my Mac. Doesn't work on my PC.
I wasn't aware of the Report an Issue. I'll do that right now.
Thanks - Will