Generally if we have problems on the bottom it is actually the top thread that's the problem but first check your bobbin. Clean out the casing. If it isn't a fresh bobbin that you wound yourself and one that came with the machine, do yourself a favor and take off the old thread and wind a fresh one. Clean the bobbin and feel that it is good and straight and no rough spots before rewinding. If the spool is fairly full, I take my manicure scissors and start cutting X's and stand over the trash can shredding it off. Try loading the bobbin both ways, that is with the thread coming off the front and from the back. Some machines don't care, others are fussier.

Some machines are designed to be fussy about how things are when you make your changes. Try making a change with your presser foot up, see if there is any difference. Make the change back with the presser foot down to see if you notice anything. My vintage machine has to have the foot down to change into zig zag mode.

Again/likewise, some are sensitive to needle position. There is usually a mark on the hand crank for when it is in the correct position (which may or may not agree to what we see/think). It can be obvious with dots and arrows or subtle, like a screw offset in the handle of an older machine, or the seam of a plastic case used as the line up point, but there is usually something there.

I googled that also and it seems like it might be a machine for leather but the information was confusing to me but it did say that it was made in Germany. Need to do a lot more searching I think.