You are very probably right that many different reasons have contributed to the figure. To which percentage deficits in the grid structure are responsible I do not exactly know. But in any case, the topic plays a big role in the German mainstream media and politics. According to what I hear, the main problem is that the industrial energy consumers are predominantly located in the south of Germany, while the best locations for wind energy are at the coast in the north. This means that Germany needs a very different large-scale electricity grid for a renewable future. Past governments were rather tardy in planning,[1] so that upgrading the grid became the limiting factor for a rapid expansion of renewable energies.

Something similar I could observe myself on a local scale in the region my parents live (also in the state of Bavaria, about 100 km away from the person who is mentioned in the Spiegel article). There, the limiting factor for the size of new solar parks was the capacity of the grid nodes in the vicinity. This is of course only anecdotal.

[1] They first wanted to build "cheap" overhead power lines. After protests from the population along the planned routes, they changed their minds and decided in favour of underground cables and had to start planing again. Unfortunately, it took more than two years for them to change their minds. (And of course, it was not them that were responsible for the delay, but the evil protesters, generally long procedures and nonsensical appeal options.)

It was Bavarian minister Aigner that in 2015 blocked/delayed the improved North-South connection. Environmental organizations opposed the connection completely despite the clear environmental benefits. https://de.wikipedia.org/wiki/Suedlink#Wilster_%E2%80%93_Ber...

The whole Suedlink process has been a triumph of pseudo-environmental Nimbyism over the actual environment and now threatens the whole energy stability of Germany in the coming energy-crunch winter.