How should I process to declare one ticket that describes the problem and to associate mutliple tickets, one for each version in which the initial ticket needs to be fixed? For example I've a bug in version 1 that needs to be fixed in 1.5 and 2.0.
Currently Easy!TTS has no feature for "propagate ticket to other releases", It is on the feature list of the comming release.
For now, you need to create another tickets which target to other releases, you can switch on reference field to track the reference relationship for those propagated tickets.
Operation to switch on "reference field":
Menu item "Site Admin" -> Submenu item "Parameters" -> Left column menu "System Fields Toggle" -> Check "usereference" on
Then, in your ticket form, you will be able to see "Tickets this refers to:" and "Tickets referring to this:" fields which can be used to tracking reference relationship among multiple tickets.
A better way of doing this should be: When you open a ticket which you want to propagate new ticket from, you will see "Clone This Ticket" on the left side (below "Operation" tab), you can use this to clone a new ticket and target it to other version.