The realtime data stream service has been upgraded and moved to a new server. The hostname is still rtserve.resif.fr but the IP changed. Please check that your clients are connecting to the new server.
To check that you are connecting to the new server, use slinktool. The output should look like this :
$ slinktool -P rtserve.resif.fr
SeedLink v3.1 (2020.075 RingServer) :: SLPROTO:3.1 CAP EXTREPLY NSWILDCARD BATCH WS:13
RESIF Ring Server | https://seismology.resif.fr | firstname.lastname@example.org