After updating to the latest version I opened and updated the library as suggested and now I have many projects with broken parts inside, from titles to effects and that's stuff I found in the first minutes. Very frustrating I tried restarting, deleting preferences and it dint work! the award part is that fcp tells me about missing titles etc but when you right click and reveal them on finder they are still there! I tried reinstalling the previous version (10.4.. I opened it and now I can't access my main library because it is updated, as it states, to work with a newer version (grrrrrr I am mad) but the titles in the browser are back to working. any suggestions or workarounds will be appreciated!
In my case I contacted FXFactory about a problem plugin and doing things like replacing the plugin, reinstalling FCPX, deleting preferences, they were able to reproduce the problem on one system and within a couple of days released an update for the plugin.
Apparently some plugins need to be updated and apparently updates not released.
I verified that both my 2013 Mac Pro on Catalina and my 2017 iMac Pro on Mojave saw the same issues.
USERS>Movies>Final Cut Backups. Dig through that folder to find an earlier version of the project and then... THEN.... DON'T OPEN IT IN ANYTHING UNTIL YOU MAKE A BACKUP COPY.
After you've done that THEN open in a previous version of FCPX and everything should be intact. If things are not quite right open A COPY in yet an earlier version of FCPX. I just looked and found I have backups on this computer going back to 2016.
One other thing I do after every final export is to save an XML file with date & time file was saved. That way you can determine if the project has retained its settings on a newer version of FCPX without actually having to update the library which, as you've discovered, poisons it and keeps you from going back to the earlier version that created it.