It seems like the problem had something to do with the computer I was transmitting to.. My good old late-2010 MacBook Pro must have had some problem with this specific board, as I didn't get this kind of behavior before with other boards, but as soon as I connected the WeMos to a Windows 10 laptop (and later to my new early-2017 MacBook Pro) - everything worked fine, just as expected!
So you can close the subject.. I couldn't find a real answer to the problem but at least I got a working solution: get a new computer...
thanks for everything
ith