WdMnl - 8:33 am on Apr 14, 2013 (gmt 0)
Indeed different time zone ..
Firstly; I assume the htaccess-file in the root is the first one WP uses, if any others are around. So, yes I replaced the WP-htaccess-file and put the old one back after I tested the new.
G1smd suggests that the given code should work as long as the redirect acts before any Wordpress rewriting code. However, with the htaccess-file replaced, having only this redirect-code it still does not work.
Anyway, I have several websites so - maybe daft that I have not done this earlier along the road - I tried the same code at a non-WP-site.
And yes (HURRAY!) it does work.
So I am overlooking something. It has probably got something to do with the installed security for WP. I am going to dig deeper into it and hope to find the solution why WP does not eat it.
And thank you both for the effort to this solution. I appreciate it. To pay back a little, I will write an article about this issue in Dutch as there is not much about this specific issue in my language. Maybe it helps others out.