There are some legitimate safety concerns for plug-in solar/battery systems, mostly related to firefighting.

With public utilities, the electrical service to a building can be cut off externally. If there's a fire, a live damaged electrical cable can reignite fires after they're put out, especially after the building is flooded with firefighting water. This also presents a potential shock hazard to firefighters. If the building has plug-in solar attached, there's no way to know which circuits might be energized or which walls might have live power in them (punching through interior walls is pretty common during fire rescue), and also no way to shut the power off from outside the building. If the system also includes a lithium battery that creates an additional problem.

Lockout/tagout for electrical maintenance also becomes more complicated, because the main breaker no longer controls all of the power for the building. Plugging in a system like this to a power outlet completely bypasses the building's breaker panel, and therefore bypasses all of the safety standards that rely on that breaker panel. It's not impossible to address, but some thought needs to be put into it and I can understand why some jurisdictions might just say "nope, we're not dealing with that."

I'm not sure how Germany is addressing these issues. I don't think it's enough to justify prohibiting the use of these systems entirely, but there is justification for establishing some safe and sane regulations around them, and not just rubber-stamping them broadly.