Start FCP with the project file that does open and do a “save as”. Choose a different name. If that doesn’t work then start making your media offline, starting with the most recent media you’ve imported. Keep going until you can save the project. The last piece of media you made offline will be the one that’s causing the crash. You’ve already trashed your render files so I don’t need to tell you about that.
If you’re still crashing after trying everything above then I’d suggest trashing the FCP pref. files to see if that’s the cause, otherwise you’ve got a really corrupt project file. Go to the Autosave Vault and open up an earlier version of your project that doesn’t crash when you try to save. You actually might want to trash the FCP pref. files first before you try anything else. This is a quick and easy way to find out if it’s FCP that’s not allowing you to save or it’s the actual project file that’s the problem.
https://www.kenstone.net/fcp_homepage/trashing_fcp_prefs.html
– Don
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