I stay logged in between pages on incognito, so I don't think that that's the case. I've never heard of this problem, but clearing my cookies has helped me the few times I've had issues on pb.
I'm not sure how to handle this on a Phone, but I had a similar problem with my computer a while back. My Disc drive wasn't working because it was jammed, so my Windows 8 thought that there was a virus preventing me from using it, so it disabled all my cookies and wouldn't let me turn them back on again. I fixed the problem, though I don't remember every step.