After 48 desperate hours I have found a solution! (at least for now).
Seems like I'm having a conversation with myself here,
but I thought I'd write this down in case someone else encounter a similar issue.
I'm not sure if this is the best way to do it or if it's even recommended, but this method worked for me.
1. Locate the corrupt Event (the Event where the corrupted file/clip lives):
- In FCPX create a clean/new Library and close it again.
- Open the corrupted Library in Finder as "Show package contents" (Right/control-click)
- Copy all files and folders plus ONE OF THE Event folders and paste it at the corresponding location in the clean Library. Click "Replace" when popup window shows.
- Open the new Library in FCPX. If it opens and the Event looks OK it means that this is NOT the corrupted Event.
- Repeat above (Always start off with a clean Library) until you encounter the Event that won't open. Then you have found the corrupted Event!
2. "Fix" the corrupt Event:
- Create a new Library again and then you create an Event within that Library that you name EXACTLY as the corrupt Event in your original Library. Then close FCPX.
- In Finder go to your original Library and "Show package contents" and locate the folder of the corrupted Event. Copy all folders within the Event folder (Original Media, Render Files, Transcoded Media) EXCEPT the CurrentVersion.fcpevent file and paste them at the corresponding location in your clean Library.
- Then go to the backups of original Library (Default: Home/Movies/Final Cut Backups) and locate the latest backup, open as "Show package contents" and locate the CurrentVersion.fcpevent file in the corresponding Event folder and copy that file to the same location in your new Library.
- Open new Library in FCPX and the corrupt Event should open.
NB. This isn't a solution for finding and removing the corrupt file, but at least now I can access the work I've done.
Hopefully some smart people will come up with a tool that can restore corrupt Libraries. Like Digital Rebellion's "Pro Maintenance Tool" which as far as I know doesn't repair FCPX-libraries.
Hope this made sense and can be of any help.