I've managed to narrow it down to a browser problem.
I tried the new browser Microsoft Edge and it worked partly. It came with a error from the PHP server saying that the upload function had a fail, but despite this error I decided to press the submit button and to my big surprise - it worked.
I've never had a problem with my trusted Firefox browser before now. So the obvious question is now whether this is a browser problem or a programming problem at the PHP server !!!.
I know from my time as web programmer and administrator on several web sites how little it takes to mess up some script code and nothing is working.
I hope you find the cause to this problem as I wouldn't change my browser to another.