6J old house with plaster cracks / bathroom tiles cracked = construction defect?

These are already cracks of a special nature... but hard to claim as a defect against the builder. However, as a buyer against the seller, no problem.

means; if the house collapses, can one sue the seller?

No, nothing will collapse, I meant that you can deduct the costs for the defect repair from the purchase price if you negotiate this accordingly with the seller. Such cracks would bother me – the 1000€ for the repair won’t hurt the seller now either.

Here are the remaining cracks. The crack in the bathroom is, as mentioned, the biggest.

If it costs 1000 euros to fix in the end, that's okay.
Sending an expert quickly will be difficult

Well, I'll put it this way. You can't necessarily expect such cracks with: aerated concrete, sand-lime bricks, aerated autoclaved concrete. However, with Poroton it's almost everyday occurrence that the hollow bricks settle significantly (what I've seen so far). Not infrequently I've been in houses where you could almost fit a finger between the drywall ceiling and the Poroton wall half a year after moving in.

Tap the tiles off, check behind and replace. That should basically take care of it. In the other rooms it's actually harmless. Stick on fleece, paint and done.

Hollow bricks settle significantly in some cases (from what I have seen so far). It was not uncommon in houses where, half a year after moving in, you could almost stick a finger between the drywall ceiling and the Poroton wall.

But it was then extremely botched and cheap somewhere. We also have hollow bricks, the house has been standing for about 3 years - there is no crack and no gap.

Yes, I have been in such houses as well. It does not mean that all are like that. It depends on a bunch of factors. But larger cracks are not really unusual in Poroton.