"If you have a look into the web and into other electronic publishings, many many authors state, that there IS a RS232 specification about maximum cable length: 50ft!"

Unfortunately, you can find plenty of rubbish on the web! :-(
http://www.8052.com/forum/read.phtml?id=46426

"And up to now, for me it was a fact, that this specification IS valid."

It isn't - in fact, the RS232 spec itself actually mentions 200ft (61m)!
http://www.8052.com/forum/read.phtml?id=57911

"Can so many people fail?"

As I said at the start of this discussion,
you must always refer to the source document(s)!

It can easily happen like this:

1. Fred reads the RS232 spec, and notes that, "with this cable, this driver, and this receiver, we can do RS232 up to 50m"

2. Tom doesn't bother to read the RS232 spec, but skims Fred's work and writes, "we can do RS232 up to 50m"

3. Dick doesn't even know that there is a formal spec for RS232; he overhears Tom and writes, "you can do RS232 up to 50m"

4. Harry vaguely remembers Dick's work, but doesn't check the reference before publishing on his website, "RS232 specifies a maximum length of 50m"