Thanks, dusktrader!
Setting PRELOAD doesn't change anything, when I reduce LookBack it works - but for my strategy I need more data. The resulting statistik I want to examine is not enough significant. Even my previous range of LookBack is too small - was only a first test.
The gap seems to be in march 2013, so I tried the history from the opserver. Same result.
Is there a possibility to suppress this error (I didn't find it in the manual)? Possibly the resulting fuzziness is less bad than a short history - worth to try.