Originally Posted by
jsloan
UA does not allow online check-in if there is an earlier partner segment still on the reservation. OP can check in online once the coupon is lifted -- generally, at some point after the partner boarding pass is scanned, depending on how quickly that information gets passed through to UA.
With LH it's pretty quick. Returning from the EU I usually do an overnight stopover in FRA and I am able to check in for my UA flight by the time I get to the FRA Hilton.