Several German makers copied and borrowed from Singer's 15 pretty early on, Husqvarna and Bernina was among them. I think they started turning up in the early 1900s. Some prewar Japan models are of this derivative too. I guess the main common feature is the oscilating hook and bobbin case at the left end of the machine. The German CB hook machines had feed dog drop and stitch lever with reverse before Singer introduced the same features on their 15 and 201. By 1940 the CB hook models already had decades of develpment and various features added by several makers. My point is really, your Alternburg doesn't necessarily have to be that close to any Singer 15 sub model. There are Singer CB hooks with the top tension dial in front too, not very common, but I have seen them on the web. The CB hook and bobbin case are 15 class, but the machine model is given a different number.
I can't offer much help with the feed dog setting, but before you start interfering with screws and adjustments, I would patientily oil, wiggle, oil, let it seep in over night, oil, wiggle, and see if anything frees up.