Why does my multimeter's overload indicator come on during normal use?
The behavior you are seeing is pretty typical.  When there isn't any voltage connected, a typical meter would set itself for the 0-299.9mV range (it has no idea whether you'd be interested in e.g. a 0.4mV signal, but the only way it could display such is at the 0-299.9mV range).  When you connect 230V, the meter will start by trying to display 230V on the 0-299.9mV range.  It will be too big, so the meter will show "overload" and its next reading will try to display 230V on the 0-2.999V range.  That won't work either, so the meter will show "Overload" and switch to the 0-29.99V range.  Again, "Overload", but then it will switch to the 0-299.9V range.  That reading will be within bounds, so the meter will display it.

Some meters are designed so that they can switch between ranges very quickly, so as to minimize the time that an "overload" indicator is shown.  On the other hand, momentarily displaying an "OL" indicator may help draw a user's attention to the fact that the decimal point is moving.

In any case, with many meters, if you wish to avoid the annoyance of having the meter change ranges, you may be able to manually select the range the meter is supposed to use.  This may be very handy when trying to observe intermittent voltages or currents.  Having the meter start out on the 0-299.9mV range when a 240V input is connected won't damage the meter, but will increase the time required before it starts showing a valid reading.  Selecting the 0-299.9V range in advance will make it possible to measure pulses which would be too short to measure using the auto-ranging function.