The German Constitutional Court in a decision (2 BvR 739/17) as published today declared the German Act for the implementation of the UP system invalid. The Court stated that the Act would factually amend the German Constitution. This would have required a two-third majority in the German parliament, which was not reached. Only 35 members of Parliament were present when the vote was held.
Interestingly, the decision was not unanimous. Justices König, Langenfeld, Maidowski published a dissenting opinion.
The press release of the German Constitutional Court can be read here (German only).
The Bundestag could now possibly remedy the situation by voting on the Act again with a two-thirds majority, but this could again significantly delay the UPC process. A more detailed analysis will be posted shortly.