I had the same problem with mine. The cool cabinet was set to +4° C and the freezer to -25° C. When I spoke to Braun, they told me it only needs to be at -18° C. So the cure was to completely de-frost the freezer, as the rear cooling unit also ices up. Make sure the pipe from the rear drip hole is completely clear. Since then, no problems.
Good luck with yours. As already suggested, a freezer thermometer seems to be called for. Just numbering the dial 1, 2, 3, 4 etc. doesn't help that much.