When I read the new plot card ' Common Cause ' I wondered when exactly the claim value is determined.

As I understand it, it is more like a dynamic concept. When someone wins a challenge, he/she looks at his/her plot card, takes that value, adds or subtracts modifiers if there are any, and then the claim effect is implemented. No problem at all.

But if you look at 'Common Cause', you cannot do it like this. What if your opponent has ' Forgotten plans ' as his/her revealed plot card? The text box of your 'Common cause' would be blanked in every phase except the plot phase. So, you have a claim value of 'X', but you cannot determine 'X' in any phase except plot phase because you have a blanked text box then.

So you would have to determine the 'X' value of 'Common cause' in the plot phase? Then every character you would play/put into play after the plot phase could not add his house affiliation to your 'X'-value. This would work, but it would be neither fair nor logical in my opinion.

What do you think about this? Did I miss something important (like an entry in the FAQ where the exact time to determine the claim value is described; I could not find such) or is there really a problem here?

When X is not fulfilled it is zero - so when your opponent has Forgotten Plans and you have Common Cause, your claim value is zero

Ignoring Forgotten Plans, as it is, X is a value which is continually checked ...

So if I kill one of your contributing characters before the challenge resolves, the claim would be reduced by 1

Well, more accurately, X is calculated from scratch each time it is needed. In this case, X is needed when initiating a claim effect or if some card effect specifically refers to the claim number on your plot card.

What do you think about this? Did I miss something important (like an entry in the FAQ where the exact time to determine the claim value is described; I could not find such) or is there really a problem here?

As Hast mentioned, the "something important" you missed was the FAQ entry that says "if X cannot be calculated, use 0." So the answer lies in how to determine any X, not in some unknown timing rule for figuring out what the claim value on your plot card should be. Everything else you described about normally calculating the claim value at the time you deal with claim effects (ie, "when you need it") is spot on. You just missed the fact that if there is no way to calculate X, the rules say to use 0 as a default. WITHOUT that default, you're absolutely correct that there would be a problem.

I was a bit confused about HastAttack's post at the first moment, because there was not indicated why X should be 0 by default. I really did not notice this entry about default value of X in the FAQ, that clears it up of course, thank you for pointing this out Ktom.

Thanks to the both of you!