A common assumption about this message has long been that some sort of mistake was made somewhere, probably not in the machine but in the setup of the machine, transcription of the ciphertext, etc.
Some of the other very long-lasting Enigma messages turned out to have probably been transcribed incorrectly either by the original intercept operator or someone down the line, for example---they decode if you swap out certain similar letters. This is particularly likely since a lot of key material and other useful documents have been recovered post-war to help with the analysis.

This particular message is particularly interesting due to its origin... U-534 is a German U-boat that was, unusually, recovered after the war. It's now a museum piece. While it was full of water that did a lot of damage, there were some documents on board that were still legible... including the Schlüsselzettel for this message, which is basically a worksheet used by the radio operator to decode messages. It's linked to from the article, along with Dan Girard's theory of why this particular Schlüsselzettel was filled out in an unusual way and missing plaintext... the radio operator at first made a mistake, and then realized that the message wasn't actually intended for (or decodable) by them, so they set it aside.

U-534's recovered documents lead to no small number of interesting Enigma messages, and the linked website has a big focus on them. They specifically reject the claim made by some that U-534 had a specially modified or possibly even broken Enigma machine, since other messages from the boat decode properly.

Enigma cryptanalysis is a very interesting hobby, since the Enigma machine is complex enough to be formidable but still simple enough to be amenable to human attacks. It's also a bit challenging from the perspective of someone used to modern cryptography, because the operation of the rotor machines is very different from modern cryptography, and because both older and modern writing about them uses a lot of terminology and methods developed at Bletchley Park that is not often used elsewhere---it seems to have been the culture of British intelligence, or at least the code-breaking type, to name things after inside jokes. This leads to odd things like Banburismus and Testery.

The Enigma was not the only rotor machine in use by the Germans during the second world war, and not the most advanced either. A more complex evolution of the rotor machine concept, code named "Tunny" by the British, was used for fixed radio links (it was too large and delicate to be practical for naval applications), and Bletchley Park mounted a huge effort to decode it was well, which lead to the development of an electromechanical computer called Colossus. In the essay collection "Colossus," edited by B. Jack Copeland, authors contend that Colossus is a major milestone in computer history, more significant than the Bombe designed for Enigma messages, which has been largely overlooked by historians because most documentation on Colossus remained classified until 2000.

Most interestingly, one author suggests that Colossus was an important precursor to the design of ENIAC which went uncredited because of its classified nature - but several key designers of ENIAC had also been designers of Colossus shortly before. This positions Colossus as a bit of a "missing link" in the development of the programmable computer, since Colossus was modified with limited programmable features similar to ENIAC's more flexible capability.

The 1970 speculative computer film "Colossus" seems to have been an amusing coincidence as knowledge of Colossus was still almost entirely classified at the time.