You can trademark any shape. However by not protecting their trademark over the years (if they every had one - which I doubt) they lost it.

It can be. But in this case they haven't defended such a case for decades, so it seems a court should throw that argument out.

Design rights[1] are a thing. However they need to be continuously defended. You can't let competitors make your designs (without license) for decades and then suddenly turn around and try to enforce the right, as seems to be the case here. Plus in the EU there are overall limits, apparently 25 years.

This is why I'm asking what the legal basis is for this case. It seems unlikely to be legally sound. Probably the German court made a mistake, and the company being sued should ignore Fender. (Not legal advice!)

Edit: Someone else just posted that Fender is now owned by private equity, so it's the usual PE playbook. A sad end to a famous brand.

Edit#2: Seems like the German court ruling was a default judgement because the other party failed to show up. So nothing to see here. Fender has no realistic case.

[1] In the EU: https://en.wikipedia.org/wiki/European_Union_design