Okay, a tech from LG was at my place on Monday, he wanted to replace the main board, but the one that LG ordered for him, was for a different model (no comment...), so he is going to come back next Monday.
Anyway, he observed the problem, and he also thinks that is a software related issue, but first he has to replace the main board, to exclude the hardware problem.
My LG LHB645 home cinema set was released in mid 2015, but it is still in production (I bought it two weeks ago, so it is new), but the last SW update was in 2016.
My TV is a very new model, with a very fresh software update (not even a month old), so he thinks that TV "speaks" to the receiver too fast, the two softwares are not totally compatible. If it is really a software issue, than it's a bigger problem, for us. If not enough customers are complaining about it, or LG doesn't experience this issue in the factory, than there isn't so much hope that they are going to fix it. Actually, I'm very angry, because I specifically bought the LG home cinema to avoid compatibility issues....
He suggested to return the home cinema, and try to find a 2017 model, but I've just realized every company started to offer just soundbars, and no HTIB 5.1 systems (of course I can buy speakers and a receiver separately, but that would be really expensive...).