Traefik 2.3.0 not picking up Docker-container

I'm using Docker containers to run Synapse, a homeserver for the Matrix protocol. One container runs the primary process while some others run so-called workers.

Everything was working fine till about five days ago when I updated to Traefik 2.3 (curiously a downgrade does not fix my issue, though!).

Since then, while the primary container is recognised just fine by Traefik via the labels in my compose file, the workers aren't recognised anymore at all. They don't appear in the dashboard nor can they be routed to. The containers themselves are running just fine and are functional, though.

I'm completely unable to see what might be wrong and I've been experimenting for days now. So, I'm hoping you'll see what I'm obviously not spotting.

Here's the non-functional label configuration:

    labels:
      - traefik.enable=true
      - traefik.http.routers.mediarepository.rule=Host(`mailstation.de`) && ( PathPrefix(`/_matrix/media/`) || Path(`/_synapse/admin/v1/purge_media_cache`) || Path(`/_synapse/admin/v1/room/.*/media.*`) || Path(`/_synapse/admin/v1/user/.*/media.*`) || Path(`/_synapse/admin/v1/media/.*`) || Path(`/_synapse/admin/v1/quarantine_media/.*`) )
      - traefik.http.routers.mediarepository.tls=true
      - traefik.http.routers.mediarepository.tls.certResolver=le
      - traefik.http.routers.mediarepository.service=mediarepository@docker
      - traefik.http.routers.mediarepository.priority=400
      - traefik.http.services.mediarepository.loadbalancer.server.port=8085

Here's a diff -Naur to compare the labels above the functional label configuration of the primary container:

     labels:
       - traefik.enable=true
-      - traefik.http.routers.synapse.rule=Host(`mailstation.de`) && ( PathPrefix(`/_matrix`) || PathPrefix(`/_synapse`) )
-      - traefik.http.routers.synapse.priority=300
-      - traefik.http.routers.synapse.service=synapse
-      - traefik.http.routers.synapse.tls=true
-      - traefik.http.services.synapse.loadbalancer.server.port=8008
-      - traefik.http.routers.synapse.tls.certResolver=le
+      - traefik.http.routers.mediarepository.rule=Host(`mailstation.de`) && ( PathPrefix(`/_matrix/media/`) || Path(`/_synapse/admin/v1/purge_media_cache`) || Path(`/_synapse/admin/v1/room/.*/media.*`) || Path(`/_synapse/admin/v1/user/.*/media.*`) || Path(`/_synapse/admin/v1/media/.*`) || Path(`/_synapse/admin/v1/quarantine_media/.*`) )
+      - traefik.http.routers.mediarepository.tls=true
+      - traefik.http.routers.mediarepository.tls.certResolver=le
+      - traefik.http.routers.mediarepository.service=mediarepository
+      - traefik.http.routers.mediarepository.priority=400
+      - traefik.http.services.mediarepository.loadbalancer.server.port=8085

Should you need any additional information, just let me know.