Exactly the same problem here. SLR app worked fine for months.... but problems started some days ago - when there was an update to the app itself. I use Firefox and german Telekom is my provider, IPv6 not active, no VPN. speedtest ID 60647.
In one of 20 tries the stream and especially switching videos works fine. But it fails in all the rest of them.
UPDATE: today there was another update for the SLR app - and my last session was okay. just some regular unstable stream in the vr girls section. but the rest was okay.