Even when your first leg is domestic, you cannot print a boarding pass online. You cannot use the dedicated “united.com” ticket counter. You need to get in the regular line and have someone scan your passport. The 45 minute cut off time is the cut off time as previous two posters have mentioned. 45 minute cut off time meaning you need to be at the counter and CSR is ready to print your boarding pass and issue you a bag tag. If you get to the airport 60 minutes prior to departure and there is a long line. You cannot get to the ticket counter until 44 minutes prior to departure, then count your blessings.
I recently missed the 45 minute cutoff time by 3 minutes, despite checking in online the day before. I was not able to get on my original flight. I pleaded with the SD and still no luck. I had to be rerouted to the non-stop instead of my usual two extra legs and 1,000 more miles before heading over to Asia. Luckily I did not need those extra EQM to re-qualify for 1K or my 150,000 EQM for two extra SWUs. They also had seats in the upper deck, but only in row 18 on the non-stop flight.