Yes it is. Nothing has changed.
No the path is actually ADMIN
This only happened after I upgraded to the latest version of mp3tag
Looks suspicious to me as usually the path to the temporary folder starts with c:\users\user-id\ where user-id reflects the current login name.
See the linked thread for means to check whether any other application has set the path for the temporary folder
That's the current "user" log-in name. It's always been that way.
This is the only program where I get that kind of error
Then I would suggest that you create a backup of your configuration, save that in a safe place,
unistall MP3tag
make sure that all folders and settings for MP3tag are gone
install MP3tag again,
restore the configuration,
and see whether the problem persists.
But perhaps that is not really necessary,
Still - as MP3tag does not complain on start-up when it creates the temporary folder and a file like _dfile_._s_ it would mean to me that MP3tag can access the temporary folder.
Could you check that the temporary folder exists as soon as you start MP3tag?
So it would be interesting to find out who generates the error message: is it a feedback by the player?
I just tried to open the temporary M3U file manually with a different player than the default player - and that works.
Can you do that as well?
To make that clear:
You are working on a local account with the user-name "admin"?
Is this the only user-account on your windows-installation?
Was your player installed with another user-account?
If you tell Mp3Tag to play some files the way you do, things happen this way:
There will be created the folder
C:\users\username\Appdata\Local\Temp\Mp3tagversion
an in this folder a temporary playlist mp3tag.m3u.
After stopping Mp3tag this folder will be deleted again.
You can open an CMD command line window and type
whoami
and press Enter
You can open an CMD command line window and type
net user
and press Enter
to get a list of local users
You mean a profile file to back up all my settings? How do I do that?
Yeah, it looks like I'm gonna have to do that. I'll use Revo and delete any registry settings mp3tag leaves behind. Then do a fresh install.
I was hoping to avoid doing that, but if that's the way it is then that's the way it is.
I'd still like my old settings back. Unless it recreates the error again.
I mean that what is described in the documentation:
Ok, I see that at the bottom of the page. Good to know.
I'll use the Standard installation and make sure any previous settings is unticked.
Once installed then I'll import that zip file backup like it says up above in the documentation..
Does that sound right to you?
Also, make sure that you have saved the configuration and stored the resulting zip-file in a safe location prior to the de-installation.
I can use a USB drive for that.
I've now done a test with different user accounts because I still suspect that this is the cause.
I usually work on a local account and I only use my Microsoft account on rare occasions.
So I logged out of my normal account and logged in to the MS account.
Mp3Tag could be accessed in the currently installed version, but the configuration was of course different, as I have never accessed Mp3Tag under this account and have not copied any configuration data.
On my normal local account, I set VLC as the default media player. On the MS account, the standard player was different (default after Windows installation).
When playing a playlist in the described way in MP3Tag I received the exact same error message that caused the thread here.
After setting up VLC as the default player in this account, playback worked without an error message.
I've uninstalled and reinstalled mp3tag and the right mouse click now does work and the AIMP player now pops up with the selected playlist as it should. That error message has gone. 
Only thing now is it doesn't automatically play when that happens and I have to manually hit the play button. No big deal, but I want that fixed since I have autoplay selected in the AIMP settings.
Couldn't this question be better answered by the AIMP specialists in the AIMP forum?
Yes and that will be the next place I go.
I was just following up this long topic with what happened, that's all.
Thank you.