Not just 'the' problem, but *THE BIGGEST* problem with any Event/Challenge/etc that requires Co-op adventures, especially ones with a shortened timeframe. PvP and Co-op are two of the worst aspects of this game and one that many players simply avoid. Until that issue is fixed, making Co-op a requirement for success is forcing players to beat their heads against their keyboards when they should be able to enjoy their gaming time.
Not all of us sit in front of our computers all day gaming. Not all of us can find people willing to run Advs at our level in our time online.
Sure, give a bonus for Co-ops, but make it so players can still see a similar level of success with multi-player Advs if they're willing to run enough of them.