UA always is allowed and able to assign seats on LH flights as long as the general parameters that Rudi pointed out are met.

LH can assign seats as well for all flights as long as the general parameters are met AND the flight is booked as a LH flight-number. However if it is booked as a UA codeshare under UA's flight number, all they can is SEE the reservation but in no way touch it until checkin.

Checkin for LH's flights usually starts around 6pm the day before it's departure and can be done in numerous ways. Elites can check in by phone, web or using their mobile phones, and everybody can checkin at the airport at that time for the next day's flight using a service called "Late Night Check In". While it is intended for morning departures (until ~11am, depending on airport) only usually people will also let you checkin for all flights the following day.

This, tom911, is probably the reason why two hours before departure the last row was the only choice for a window seat as by that time quite a few people had already checked in. Every connecting passenger, Elites using their options and several "Late-night"-customers had checked-in by that time and taken up all desirable seats with just a few FRA-originating passengers left.

Did you perhaps notice your boarding-sequence number? I bet it was a high two-digit or even a low three-digit number...

Greetings - Dirk