Normally 48 hours to the exact second but I also had trouble checking in online yesterday and it was a good couple of hours later before I was able to complete check-in so keep trying.

I think once before on here someone commented that your connecting flight should be within 60 hours of your check-in for the first segment (although I haven't personally encountered that before) - could that be your issue?

Otherwise probably just some gremlins in the system!