@ohrenkino and @DetlevD
Thanks a lot of quick answer.
Maybe original the playlists only support 7-bit ansi character set.
When do you use french and german special characters like à, â, é, è,
ä, ö, ü, ß etc. there will be no problems. All this chacacters are above
the value 127.
Yes there are the usual rules of the file system depend of the first days
starting with CP/M and DOS.
The danger of mis-interpreted could be. But since bad programs (virus,
trojan etc.) take advantage of this mis-interpreted folder and file names
the programmer of software specially programmer of Microsoft spent many time
to remove this malfunctions. I think the danger of mis-interpreted do not
really exists now.
By yourself you can generate any folders and files with different combinations
of special characters. Usually there are no problems because Windows can use
different languages with many special characters like arabian, asian etc.
(ASCII, Ansi, Unicode).
For western people the discussed characters are special but most of the users
in the world use "special" characters. For this users their used characters
are usual and not special.
With your update of your post I realize that you cann't reconstruct the
problem of no save the m3u file.
Many luck I found the reason.
I have some operating systems in virtual machines of Virtualbox.
I copied and installed MP3TAG, Foobar2000 and some mp3 files in a virtual
machine of 'Windows 10 Pro Build 10586.164 Version 1511 x64 de'.
In a virtual machine of 'Windows 7 Ultimate SP1 x64 de' I do the same.
Et voilà, in the virtual machine of Windows 10 MP3TAG can save the m3u file
in the same folder without a problem. In the virtual machine of Windows 7
MP3TAG shows an error window 'Die Datei ...\mp3tag.m3u konnte nicht angelegt
werden.' when I use the function Datei\Playlist (alle Dateien).
In this brackets you have to imagine many bad words and bad emojis.
Of course my host and working operating system is Windows 7.
Ok, I think the problem is less a problem of MP3TAG than a problem of the
operating system Windows 7.
Most of the programs usually use the functions of the operating system to
handle the operations of files and folders.
At the latest in 2,5 years most of the users will use Windows 10 because of
the end of support of Windows 7. Then the problem is obsolete.
I wait since Microsoft will implement the promise the function updates will
come like usual upates and not with a installation of a whole operating
system. I hope I don't wait my life time.
PS: Many greetings from Wuppertal to Wuppertal. The world is very small.