As soon as more direct / online banks join the EBA SCTinstant most of the Sparkassen become finally obsolete if instant was personal requirement. Hoping that these new members will use sophisticated realtime transactions monitoring systems from the beginning. HVB and Deutsche Bank are the only large private banks in Germany currently supporting instants and they are known to use high end real-time transaction monitoring systems, at least one of them.
I don't know, its only speculative, but what if the Sparkassen doesn't use the latest state of the art tools to detect any security concerns regarding transactions. With normal SCTs there is more "time" to check by the old fashioned way. With instants, especially during off-business hours / weekends you purely rely on high end software / algos / AI. So if the Sparkassen would have these problems they would protect themselves and shut down transfers with the other bank or establish low transactions limits (if they can, SCTinstants are irrevocable). And they would do this relative to the number of issues they have. There are 100s of Sparkassen, on one side acting as a group (also via a Landesbank like the RT1direct participants Helaba and Baylaba) but on the other side on their own, having different connections, directly or indirectly with the giro system. They didn't start form the scratch like all fintechs so they might have a challenging mix of IT that had to adjust to the new world without being able to do so easily.
What could be the "security issues" one Sparkassen customers has gotten as explanations from their account rep regarding the instants? If you surf the web you will notice that instant payments like faster payments in the UK and other systems around the world face an increase in misuse. What make it attractive for the criminals is that its instant. They immediately get the money in their hands. And the way they do it is most likely with fishing. All Sparkassen have 50 Million customers in Germany...by far the largest (instant) group in Germany. The large the number of accounts and a probably large share of customers no very familiar with fishing threats could results in a number of issues. These are just my thought. I don't have any real evidence, but might be an explanation.
Anyway...If thats all the case this process might take some time to fix. Hopefully bunq can help them or find a common solution. As they told, they are working hard on this and speak to the Sparkassen.
It would be very helpful if anyone faces similar instant problems with any non Sparkasse, either Deutsche or Hypo or even other European Banks. I heard of some austrian customers (Austria has a large number of Sparkassen connected to EBA instant system) dont have problems sending money back on forth instantly with bunq. Right now it seems to be a purely german Sparkassen issue. Please post if there are changes with others.