Hello Andreas,
Thanks for your message and advice. That might have been the cause of the problem.
I changed my theme and that was not a solution to it.
I have now finally solved the problem. I went into Site Administration > Server > HTTP
The first item on that page 'Use slash arguments' was selected. I deselected it, and cleared the browsing history on my browser (clear cache). Voila, that solved it.
Once again, thanks.