[mkgmap-dev] mkgmap and "memory-full-error"

Hi! I've downgraded my Oregon 450 from firmware 6.xx to 5.50, because this was the last firmware without the activity routing. With the activity routing the routing of my maps was broken (car routing over steps and ignoring of "access=no"). With the firmware 5.50 the routing was correct again. But after a few weeks I get the famous memory-full-error. Now I read that in the past several people reported this error with maps made with mkgmap. The only advice I found, was to use an recent firmware. Is there a known bug in mkgmap? How can I avoid the error? Christian