Could you please check File settings in Export dialog box?
Please post a screenshot of the option available for the Image format?
Also, please check and confirm the version of Lightroom are you using?
Which operating system are you using?
Thank you for your quick answer! In order to create a screenshot for sendigt it to you, I opened Lightroom 6.0 and the Export Dialog box. But surprisingly to me the Export box now showed the Option for the Image Format. This was not the case during all my former attempts today. I now believe in miracles. The Problem obviosly was solved without any Action from me. The Export was succsessful, I do not know why now. Problem is solved! Thanks!