Thanks to all replies and suggestions, which brought me on a way to a solution.
(I didn't get any info from phototherm on this one, no reply on two emails. Sorry.)
I cut "sqares" about 5cm x 6.5 cm (wide) out of a smooth plastic soda water bottle (1L), just slightly wider than a film/the reel opening, the bend of the "bottle" in line with the bending of the reel. Two small cut-outs in the sides clip the plastic sqare to the first spokes and keep it firmly in place. The "squares" are pushed into the reel, on top of the film end, overlapping it by at least 3cm (up to 4cm). No more issues since then, even in extreme test runs.
This is for Hewes SS reels. An almost identical approach would work with Paterson / AP plastic reels.
Again thanks to all for the helpful hints.