Hi, thanks for your reply.
I found out the issue. My file size was too big (in my php.ini file I declared 2MB, but the file has 4MB).
Just strange with the error message.
I just took a look in the source code and found out following:
In the file kirby/vendor/getkirby/toolkit/lib/upload.php in line 193 it throws an exception ($messages = $this->messages()).
For me the function messages() looks good, it just returns a normal array with some error messages.
So instead of throwing the error with the correct error message, it throws the message "a non well formed numeric value encountered".
Maybe it´s windows related?
My server is running with PHP 7.1.5 under FastCGI mode.
Maybe some of you Pro´s can find out why the function is throwing an exception only at windows.