Epiphany fails to obtain Firefox Sync certificate

I'm using Epiphany 46.0 (WebKitGTK 2.44.1, GStreamer 1.24.3). As soon I start the web browser a tooltip is shown with the message Failed to obtain signed certificate. Please visit Firefox Sync and sign in again to continue syncing. Debug information with EPHY_LOG_MODULES=ephy-sync-service.c shows:

~$ EPHY_LOG_MODULES=ephy-sync-service.c epiphany
[ ephy-sync-service.c ] Syncing tabs collection initial...
[ ephy-sync-service.c ] Syncing passwords collection initial...
[ ephy-sync-service.c ] Syncing history collection initial...
[ ephy-sync-service.c ] Scheduled new sync with frequency 15 minutes

(epiphany:7857): epiphany-WARNING **: 12:28:22.467: Failed to sign certificate. Status code: 404, response: {"code":404,"errno":116,"error":"Gone","message":"This endpoint is no longer supported","info":"https://mozilla.github.io/ecosystem-platform/api#section/Response-format"}