FF Mobile not able to sync

[F0x0185]

Hello,
since several days I try to get my custom Mozilla Sync Server (v0.17.15-3 for 6.2 apollolake synology) running with the latest Android FF Mobile client.

On my desktop FF 134.0.2 clients everything is running smoothly with and without SSL.
On my Pixel 8 with Android 14 and the 134.0.2 (Build #2016068999) the initial connection works but then nothing is synced.

I tried it with an HTTP and HTTPS uri (with reverse proxy) with no luck.
Therefore I used putty to get access to the sync server logs.

I can see the initial OAuth token connection and this connection was identically repeated approximately each 10-12 minutes. Sometimes I can trigger it by opening the sync settings on FF Mobile.

e.g.

Jan 31 21:09:38.073 INFO {"token_type":"OAuth","metrics_uid":"0c8888ed25fcc5fd5c3052914127373f","ua.browser.ver":"134.0","first_seen_at":"1738185123258","ua.browser.family":"Firefox","ua.os.ver":"14","uri.method":"GET","ua.os.family":"Other","ua.name":"Firefox","ua":"134.0","uri.path":"/1.0/sync/1.5","uid":"0c8888ed25fcc5fd5c3052914127373f0359a4e75e061c63f8888ff1e4b3c0d2"}

The uid was also included in the {"Type":"glean-server-event", ...} token.

Perhaps you can give me a hint how to get this setup running. Mozilla support point at the sync server and gives no support why sync on FF Mobile is not working properly.

Thanks!