I have just installed IDM and it tells that my trial period expired and asks for serial number. Why does it happen?

Probably you installed old trial version of IDM before and your trial period has already expired. IDM shares the same trial period for all versions. The only way to use IDM now is to purchase the full version. You can then use it for 30 days and if you decide that you do not need it, we will refund the order.