Hi Trentiano
This can be caused by several things.
Corruption in User account. Don't think so because you can open other Libraries/Projects. You can try to create new User account (Admin) and open Library/Project. If new account will open project then you most likely have a corruption in your User account.
Corrupted or bad/non compatible plug-in. This can be caused if you added a plug-in recently or if you upgraded OS or software versions and created conflict. Did You??
Corrupted file/clip. This is very possible. Even when initially added/imported, maybe fine, but placing in Timeline causing problems, especially when FCPX is launched and needs to read file. This is sometimes overcome when opening project in new User account(not always). Copying contents over, results in same issue.
You can try to open new project, import half of old content and see if it crashes. Then delete that content add the other half and narrow down.
The best way of figuring issue maybe in System>Library>Logs>Crash Reporter or Diagnostic Reports. If actual crash there will be file created for the crash and will point to problem. It is also possible to use information in "Console" (Utilites Folder).
There is also EtreCheck a free Utility that will analyze your system for possible problems.
You can also delete FCPX preferences by launching FCPX while holding down the Command and Option keys. Delete preferences in pop-up and see if FCPX will open your project.
Hope this helps, Greg
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