I try to export my timeline content (between in and out) to Quick Time Movie,
but it doesn't work anymore. Only the first clip is exported, not the whole timeline.
It worked before, I'm sure of that. I must be doing something wrong.
De-select all In-Out points and try again. If you're trying to export the entire timeline, there's no need to set In-Out points. Also, are you trying to export using a plugin like Flip4Mac? Trial versions of some of these will restrict the export so you can't just use the free version for work.