404 for files with special utf-8-chars

I can't play files with special utf-8-chars (eg Umlauts like "Ä"). In the whole UI (and Django admin-interface) they show up correctly. But when I want to play the track it just skips.

The nginx-log gives some more insight into this:

nginx_funkwhale_1  | 2018-05-09T20:33:45.400822121Z 2018/05/09 20:33:45 [error] 6#6: *90 open() "/srv/funkwhale/data/music/sorted/Die �rzte/13/2-05 Liebe und Schmerz.mp3" failed (2: No such file or directory), client: 172.21.0.10, server: <server>, request: "GET /api/v1/trackfiles/3412/serve/?jwt=<jwt> HTTP/1.1", upstream: "http://192.168.48.3:5000/api/v1/trackfiles/3412/serve/?jwt=<jwt>", host: "<server>", referrer: "https://<server>/library/artists?query=&page=1&paginateBy=12&ordering=-creation_date"
nginx_funkwhale_1  | 2018-05-09T20:33:45.402979423Z 172.21.0.10 - - [09/May/2018:20:33:45 +0000] "GET /api/v1/trackfiles/3412/serve/?jwt=<jwt> HTTP/1.1" 404 170 "https://<server>/library/artists?query=&page=1&paginateBy=12&ordering=-creation_date" "Mozilla/5.0 (X11; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0" "46.246.61.160"

Something is messing up the encoding and replacing the "Ä" with a "�".

I checked and double checked my nginx-Config - it should handle utf-8 correctly (everything there is set accordingly).

I imported the files in-place on my existing collection. Other files (without special chars) play correctly - any files with umlauts or japanese chars in the filepath get messed up.

I also checked the files themselves and the title is correctly encoded in utf-8.

May the API-Container be messing up the filepath?