Well, I'm having fun while "testing" thanks to your work ;)
I'm now focusing more on side quests now as you suggested, I'll write down everytime one npc sends me back to main menu. Hope this helps finding the root cause for the quest problem. Having done more quests where everything worked fine than quests where I needed to go back into the game. I'm doing them in no specific order. I'll keep updating the list as I play more.
When savegames are included they're in front of the target npc and after loading back into and clicking on npc it always results on my machine in get kicked back to main menu.
http://www.sacredwiki.org/index.php/Sacred_2:Children_and_Demons
- When talking to Farmer Koremar after escorting the children (done this quest with two chars, happened only with one where I needed a second try)
http://www.sacredwiki.org/index.php/Sacred_2:The_Lost_Amulets
- When talking to Nimantus for reward
http://www.sacredwiki.org/index.php/Sacred_2:Tragic_End
- When talking to Nimantus for reward
http://www.sacredwiki.org/index.php/Sacred_2:The_Runemaster's_Son
- When talking to Zamanar for reward (2 times in row) (tried quest with second char, worked fine first try)
http://www.sacredwiki.org/index.php/Sacred_2:Children_and_Demons_II
- When talking to Elodriel for reward
no wikipage exists, guess its a reenabled quest: the bride who doesn't dare
- When talking to Blind Guardian Fan in Thylysium for reward
random encounter
- When talking randomly to Alan Wild for no specific quest (had ! on his head anymay)
http://www.sacredwiki.org/index.php/Sacred_2:Changing_Sides
- When killing Orpheus (the moment he dies)
http://www.sacredwiki.org/index.php/Sacred_2:The_Elixir_of_Souls
- When talking to Retilynen for reward
http://www.sacredwiki.org/index.php/Sacred_2:Ghost_Hunting
- When talking to Retilynen for reward (4 times in row)
http://www.sacredwiki.org/index.php/Sacred_2:Leader_of_the_Pack
- When talking to High Elf Soldier for reward (needed 13 tries, even restartet game meanwhile, teleportet away, did another quest and then tried again and it finally worked) (savegame)
http://www.sacredwiki.org/index.php/Sacred_2:The_Hunt
- When talking to General Whimsical for reward (2 times)
http://www.sacredwiki.org/index.php/Sacred_2:An_Eye_for_an_Eye
- When talking to Guardsman for reward (10+ times) (savegame)
http://www.sacredwiki.org/index.php/Sacred_2:Good_Address
- When talking to Captain Hank for reward (10+ times) (savegame)