240 V instead of 230 V on a true RMS multimeter
Is it possible that my meter is wrong or is it a consequence of a true
  RMS measurement?


It shouldn't be an issue. The sine wave purity of most "mains" supplies is usually pretty good and this will be registered correctly on an RMS meter anyway. Cheaper (rectifer type) meters will start to show a discepancy as the distortion increases but this would be hardly noticeable given the "very reasonable" quality of most AC supplies but this doesn't apply to a lot of battery powered (solar) inverters.

It might be a 4.3% error of measurement or it might be that your AC supply is not quite exactly right. Read the data that came with your multimeter to find out.

According to this wiki page and over-voltage or under-voltage is classified at the 110% and 90% level.