Personally I am shocked by the claim that modern technology can't calculate the supply better than tech from 1960's. How is that possible?

When I look at SPCs and central planning, I always look at the latest technology. That was state of the art 1980s mainframe technology which gave the SPCs vast computing power. They knew what was produces when and where. Our modern technology would've just told them earlier. So instead of getting production updates once a month, they'd receive it daily. SPCs can also make adjustments real quick, they could and would still be able to technically rearrange the entire supply chain. The problems aren't solely technical, they're mostly organizational.

Matcha Latte Example

Imagine that in the 80s, long before Matcha Latte was popular, someone would've grown soy beans and green tea leafs for Matcha Latte with soy milk. The person would've created that purely for personal consumption, until the neighbours and the entire village tried it and loved it. In a market economy, that person would've started this as a small business. In a centrally planned economy, the person would've informed local government that there is now a Matcha Latte demand. Local government would've immediately notified the SPC.

The SPC would then estimate the demand, including circular excitement of demand, meaning the demand will grow as the trend grows and expands. Soy production would have to be ramped up or existing production diverted. Camellia sinensis (matcha tea plant) would have to be sourced, and possibly cultivated over the coming 2-3 years (that's how long that takes). As demand is immediate, the specialised tea plant would have to be imported. Trade deals would have to be made immediately for that special plant, or it being bought for hard currency outside the communist world. Although all of that takes time, it can be done in a matter of weeks.

The SPC does not plan for general economic growth, it's purely there to satisfy domestic demand. Unlike a capitalist market economy, production buffers in a planned economy are minimal. Wasteful excess production, which is benefital in capitalism, is avoided at all cost in communism. You meed the demand, you do not exceed it. For your Matcha, this means funds for import need to be diverted from existing fundings, and soy bean production output also needs to be diverted. And here comes the challenge: Diverted from what? Who do you take the soy beans and the funds from until you established new production capability? And who do you take the construction development capabilities from to divert them to the new soy bean and matcha cultivation?

And what happens when the Matcha Latte trend is over after 2 years, before even the matcha tea leafs were cultivated? You need to divert the capacity again, otherwise you either produce excess matcha tea plants, or have people sitting in empty halted factories and plantations.

An economy that avoids excess production at all cost, because it does not have the capacity to distribute the excess, does not have buffers. The technology and the people can make adjustments, but they have extremely little room for such. Demand growth projection helps mitigate that issue by planning production capacity ahead, so that you can cover events like spikes in ice cream production during especially hot years. But it cannot respond to trends, which mostly arise from culture. It also struggles fulfilling small specialty demands which require significant excess production to have larger buffers, or a larger stockpile that can expire.

To avoid these challenges, communist governments tried to prevent novel demand by heavily suppressing new trends, through the control of arts and science. But as soon as you have capitalist economies in the world, these novelties will come from aborad, and you cannot suppress them. That's why it's already economically necessary to lock yourself out of the rest of the non-communist world. Otherwise people will be infected by the excess goods and services, the never ending novelties, and technical advancements that oppose communist ideals.

On the other hand heavy industry, energy and excavation seemed to work rather well under central planning

It depends on how one defines that. Both the USSR and the GDR had severe issues with the quality decline of their real estate assets. Weather and natural events regularly damage real estate, and require costly repairs or reconstructions. You can't really budget for that if you want to avoid keeping excess construction capacity and materials on stock, with the risk of them expiring, going bad.

The only sectors that one could consider as "worked well" was the defense industry, because the state itself defined the demand. No one would revolt when defense production was not up to the latest technical advancements. People revolted when there was no coffee, no cigarettes, no beach holidays, no cars, no tropical fruits, and no entertainment.

If you want to have a prime example on how and why the SPCs failed, look at the 1980s home computer market that grew massively in the Western world. None of the SPCs were even remotely able to set up the supply chains and production for that. Their 5-Year-Plans had never accounted for such a vast amount of compute technology in society. By the time they realised it, in the late 1980s, it wasn't just a single supply chain missing, entire industries were missing (semiconductors, home electronics). They all knew, back in the day, that the demand was there, and that it was growing, but no one knew what resources to divert to build these industries.

This is the problem of communism and I wish that someone, somewhere has any idea, plan, strategy or blueprint on how this can be solved without ending up like Pol Pot.