This is the same bug as was introduced with 7.4.  See: https://bugs-archive.lyrion.org/show_bug.cgi?id=15486

There is some issue with the encoding of characters, which is now different.  It doesn't matter in the MusicIP settings if you have the 'Use MusicIP' option set as "Only read mixable status from MusicIP (faster)" or "Import all song data from MusicIP (slower)".

You can create mixes with songs with accented characters but any song in the resulting mix that has 'weird' characters cannot be played.

Typical error in the log when you try to play:
[10-02-13 19:18:03.1554] Slim::Formats::readTags (157) File missing: E:\Maho Backup\Music\All Music\Music\' P Q R S T\Rubén González\Rubén González - 2000 - Rubén González And Friends\07 - Mulatica Retozona.mp3
[10-02-13 19:18:03.1561] Slim::Schema::objectForUrl (809) Error: Null track request! Returning undef.
[10-02-13 19:18:03.1572] Slim::Schema::objectForUrl (809) Backtrace:

   frame 0: Slim::Utils::Log::logBacktrace (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Schema.pm line 809)
   frame 1: Slim::Schema::objectForUrl (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Web/Pages/Playlist.pm line 164)
   frame 2: Slim::Web::Pages::Playlist::playlist (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Web/HTTP.pm line 1097)
   frame 3: Slim::Web::HTTP::generateHTTPResponse (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Web/HTTP.pm line 924)
   frame 4: Slim::Web::HTTP::processURL (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Web/HTTP.pm line 735)
   frame 5: Slim::Web::HTTP::processHTTP (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Networking/IO/Select.pm line 139)
   frame 6: (eval) (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Networking/IO/Select.pm line 123)
   frame 7: Slim::Networking::IO::Select::__ANON__ (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Networking/IO/Select.pm line 183)
   frame 8: (eval) (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>Slim/Networking/IO/Select.pm line 183)
   frame 9: Slim::Networking::IO::Select::loop (slimserver.pl line 626)
   frame 10: main::idle (slimserver.pl line 580)
   frame 11: main::main (slimserver.pl line 99)
   frame 12: PerlSvc::Interactive (/<C:\PROGRA~1\SQUEEZ~4\server\SQUEEZ~3.EXE>PerlSvc.pm line 99)
   frame 13: PerlSvc::_interactive (slimserver.pl line 0)
   frame 14: (eval) (slimserver.pl line 0)

[10-02-13 19:18:03.1577] Slim::Web::Pages::Playlist::playlist (166) Error: Couldn't retrieve objectForUrl: [] - skipping!



Typical errors when trying to create:
[10-02-13 19:21:02.3390] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/%27%20P%20Q%20R%20S%20T/Suzanne%20Vega/1992%20-%2099.9%20F%C2%B0/08%20-%20As%20A%20Child.mp3] - skipping!
[10-02-13 19:21:02.4132] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/%27%20A%20B%20C%20D%20E/%C2%A1Cubanismo!/%C2%A1Cubanismo!%20-%201997%20-%20Malembe/04%20-%20El%20Pregunt%C3%B3n.mp3] - skipping!
[10-02-13 19:21:02.4193] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/%27%20F%20G%20H%20I%20J/Gloria%20Estafan/2000%20-%20Alma%20Caribe%C3%B1a/10%20-%20S%C3%B3lo%20Por%20Tu%20Amor.mp3] - skipping!
[10-02-13 19:21:02.5172] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/VA/The%20Rough%20Guide%20To%20The%20Music%20Of%20Cuba%20-%201998/03%20-%20Celina%20Gonzalez%20-%20Santa%20B%C3%A1rbara.mp3] - skipping!
[10-02-13 19:21:02.5348] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/%27%20A%20B%20C%20D%20E/Bebo%20Vald%C3%A9s/Bebo%20Vald%C3%A9s%20-%202004%20-%20Suite%20Cubana/03%20-%20Devoci%C3%B3n.mp3] - skipping!
[10-02-13 19:21:02.5523] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/%27%20A%20B%20C%20D%20E/%C2%A1Cubanismo!/%C2%A1Cubanismo!%20-%201996%20-%20%C2%A1Cubanismo!/01%20-%20Descarga%20De%20Hoy.mp3] - skipping!
[10-02-13 19:21:02.6808] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/VA/Brazil%20Classics%203%20-%20Forr%C3%B3%20Etc.%20-%201991/05%20-%20Dominguinhos%20-%20Querubim.mp3] - skipping!
[10-02-13 19:21:02.6980] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/%27%20A%20B%20C%20D%20E/Andr%C3%A9%20Hazes/1981%20-%20Gewoon%20Andr%C3%A9/12%20-%20Het%20Laatste%20Rondje.mp3] - skipping!
[10-02-13 19:21:02.7659] Slim::Formats::Playlists::M3U::write (220) Error: Couldn't retrieve objectForUrl: [file:///E:/Maho%20Backup/Music/All%20Music/Music/%27%20P%20Q%20R%20S%20T/Rub%C3%A9n%20Gonz%C3%A1lez/Rub%C3%A9n%20Gonz%C3%A1lez%20-%202000%20-%20Rub%C3%A9n%20Gonz%C3%A1lez%20And%20Friends/07%20-%20Mulatica%20Retozona.mp3] - skipping!
[10-02-13 19:21:03.4266] Slim::Formats::readTags (157) File missing: E:\Maho Backup\Music\All Music\Music\' P Q R S T\Suzanne Vega\1992 - 99.9 F°\08 - As A Child.mp3