Should the underground cable to the garden be secured again?

So could the 30mA RCD in the fuse box also trip?

That is correct.

That would still be OK for me, because after all I can try to switch it back on first.

What do you do if it trips again immediately? Then the problem can still be outside or inside. Unless it is disconnected all-pole outside, then a trip immediately would indicate the problem is inside. The thing is that with RCDs the problems are often not so clear. Due to leakage currents, but also certain types of loads, triggering can occur with a delay or depending on humidity. Can't you just run a new cable from the distributor?

Unfortunately, as mentioned, that is not possible. If the RCD trips outside and does not trip inside, can the problem still be inside? The RCD outside does disconnect L and N from the rest of the house. PE remains connected.

Unfortunately, as mentioned, this is not possible.

Why not? Drill, cables, sealing, and the problem is solved properly and not a hack that only sometimes works.

If the RCD trips outside but not inside, can the problem still be inside?

Correct. However, it is not guaranteed that if there is a leakage current outside, only the outside trips. The statement from is simply wrong. Simple example: 10mA RCD outside, 30mA RCD before that. 50mA leakage current. Both would probably trip here (without true selectivity). And I wouldn't want to use truly selective RCDs with delay in the living room. I'm not even sure if that would be allowed.

Can he fly outside (FI 10mA) because I have about a 10mA fault current in the living room? And if both trip, if I first only switch the inner FI back on, can I not locate the problem this way (if it trips again, something is wrong in the WHZ; if it stays on and then the outer one trips when switched back on, it's the garden)?

What you describe above should also simply be implementable with a multipole switch. Where do you even want to install such an "external FI"?

What I have seen so far is a portable RCD, as a kind of intermediate plug. There are also versions with corresponding IP protection classes. However, I would even prefer an all-pole switch. If there is such a thing, preferably with a high IP protection class, then that would be my preferred choice.