Here I have a picture attached of the level. Basically, you're supposed to go to the top 3 key doors (green, red, green) before you go into the lower ones. (Red, yellow, yellow, yellow, blue) I made the mistake of trying to use the lower doors first and afterwords figured out I couldn't finish the mansion. (I used the red key on the bottom door and didn't have another red key for the top door... The top door, though, contained two yellow keys I needed to complete the lower set of doors.)

Hopefully this isn't too hard to understand! :S

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Did you explore the sky boxes type area yet? You get two red keys there which open that green/red/green door.

Yep. I made sure I went through the sky box area. I was able to finish the mansion later by going to the top door area first after restarting from the save point, but I'm fairly confident I wouldn't have been able to finish the mansion if I had gone to the bottom set of doors. I spent about twenty minutes looking for a spare red key and couldn't find one.

I have the exact same problem as the original poster. I used my last red key long ago on the lower red door shown in the picture, and now I can't find any more red keys to open the green-red-green combination on top. I wonder if I am stuck now and have to start over  :-/

Start over. It is just as in real life if you take the wrong turn go back and start anew.

Actually, I have a similar problem in Mt Peril Normal, I'm stuck somewhere with no means of backtracking. Admittedly, I may have been a bit careless in key handling (get a key + see a door = I just run to open it), but still it is a bit unfortunate if one has to restart a section.

(Although at this point I'm not sure I haven't just missed a half-hidden key or something.)

Hmm... it seems that we may have to re-configure the keys and doors a little, I must have made an error somewhere in there. Anyone have any possible solutions? This is quite the tough puzzle of figuring out a way out of this...

I don't really mind THAT much starting over, or starting from a save point, but... I don't recall having this problem in the first MM, There were some situations where you had to use the keys in the "right" order in order to access some secret or some treasure, but I never reached a point where I couldn't finish a mansion. I'm not sure what the "policy" is - whether mansions are ever allowed to be unsolvable or not.

No, a mansion should never be unsolvable. Any problems like this that weren't discovered during beta testing will be corrected in the next update.

That's a relief :)
Thanks for the info.