Today started with a benign question from Electricien Belgium, asking for news. Then popcorn intensified with a email from them telling me that it's my duty. After replying to them that, although it's a deal between themselves and the constructor, I do what I can, they went full aggressive mode mentioning lawyers again. Apparently (get more popcorn, ladies and gents), they say that their lawyer told them to tell me (you get it?) that it's my duty to resolve their problem.
After a relatively short communication with the constructor, the site manager told me:
we have a cash liquidity problem
...and asked us to unblock the deadlock by advancing them the difference.
In the spirit of good collaboration, we did so, and Electricien Belgium got their second payment.
Now, next week, there will be a meeting with everybody!