Same issue! I can't get to the quest shops at all either!
It's happening to me too-it's a relief to know it's not just me that's it's been happening too.
[tot=Azrel]
Now it's worse for me. It was just a couple of them yesterday but it's all of them today. :(
It is doing it to me as well and I even waited for a little bit, restarted my lap top and it is still doing it

Just now tried a Shinwa quest and could not complete the first one (Maleria)
Don't want to miss out on the special weekend quest giver :(
I too have an error when trying to quest. No Shinwa, Blue Building, or anyone else. Additionally, I cannot enter special shops like the MC, Quest Point shop, event stores (Vesnali, Zombie Den, Steamworks, etc etc.)
Major Drills works though, as well as Battle Quests. I guess that's all I'll get done for today.
Started Cinthia around 1/2pm today, couldn't complete (and timed out ofc). has been happening since then for me - can't do any quests at all. Windows 8.1, updated firefox.
Blue: 0 / 5 Carl: 0 / 15 Cinthia: 0 / 15 Cursed: 0 / 15 Library: 0 / 15 Maleria: 0 / 15 Pete: 0 / 15 Quentin: 0 / 15 Sarah: 0 / 15 Saggitarius: 0 / 15 Wizard: 0 / 15 Battle: 1 / 30 Major Drills: 1 / 30 Shinwa: Play now!
Just dont even feel like doing BQs, nor MD. :/
Some communication about this issue, would be nice!!!
I'm also having this issue. I'm a bit irritated since I try to do quests every day, and now I'm going to be missing out a day on 15 quests (gold account... not much help if I can't use it!)
Same error, on my first shinwa quest (mori/nori)
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Can't get into any of the main quests. Refreshing does nothing. :/
Having this issue on all the quests :(
This just hit me and I can't do any quests. Q^Q
No quests are working for me. No quest shop.
EVERY quest giver, no amount of refreshing, gives me the "this quest isn't valid!".

Quests have been fine for me until now! I was able to do Item Hunt but other other quests say they're not "valid." What's going on????