In the instructions of the Sew-along Julie Paster advised two things to prevent puckering. Firstly to iron onto the fabric a fusible interfacing. Secondly a sticky stabilizer in the hoop.
But I had no chance of buying that interfacing in a fabric shop I had to omit that and use spray-on starch instead. And I do not like that sticky stabilizer so I used tear-away with temporary adhesive. And as the first sequence of the design is a square at the outline of the design this serves perfectly as a basting stitch.
So nothing puckered. But at the writing the bobbinthread shows on top. Why this happens I have to find out. It is not a worn needle, as I put in a new one before I started. I have already checked the bobbin thread tension with no effect. Tomorrow I will try using a second layer of stabilizer under the writing.