I saved some replay numbers from around August, have not been able to watch them for quite some time. You are not the only one Planktum.
Umm, I know for huge games with huge systems and unit counts it can be hit or miss getting it to play. Also have to remember that you need to wait awhile after the game ends before the server will allow anyone to watch it. I don't know what the exact time is, but 10 minutes after the game should be long enough.
If you have the pastats mod installed, it keeps track of all of your games, and provide you with the lobby number to replay the games. But will only keep track of the games you play with the mod installed. The replay system currently very clunky.
Too bad, am missing some interesting replays as well... and I just disabled PAstats. Suddenly, I cannot connect to any multiplayer games when PAstats is enabled.