Hi all -- I'm having a similar issue and would appreciate any and all help.

I'm flying SIN-NRT-IAH on a UA ticket. Both segments have UA flight numbers, with the first flight operated by ANA and the second by UA. I'm getting an error message that online check in isn't available at this time (FWIW I'm within 24 hours for SIN-NRT, but not NRT-IAH). I'm using a US passport.

Any suggestions on how to complete online check in? I asked UA to switch me to native NH flight numbers but said they weren't allowed to. I can't arrive at SIN until uncomfortably close to departure time, so really hoping to check in online

Thanks all!