See if you can rule out the cause of the error step by step:

Try clearing your browsercache
Try changing your container skin to the default supplied with DNN (Minimal Extrophy).
Try changing you page skin to the default supplied with DNN (Minimal Extrophy).
Try to move any third party modules onto another page.
Try moving any other module onto another page.

Hope this helps.

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The file was empty.
It was the same with the other files for the previous versions.
The first installation was 5.0.1.

I note the file release.config is missing from the upgrade - is this a mistake or does one continue with the 5.5.1 web.config?
JC