Final Cut Pro X :: 10.1.3 - Yellow Triangle With Exclamation Mark On Event

I have a yellow triangle with an exclamation mark on my event. I was working on it and noticed it when I restarted FCP X. Could be that FCP X was crashing while copying or something. But now I don't get it away. But I looked into it. 

The 4 photo's and 1 piece of music were quickly found in the menu left of the timeline. But there are two strange stings going on: 

1. Scrubbing through the timeline, at certain points the Beach-ball begins to spin and won't stop. I waited 30 minutes (nothing), restart FCP X (nothing), rebooted (nothing), cleaned with Onyx (nothing): I had to force quit FCP X. I cannot see where it goes wrong, but it's at multiple points. 

2. While using "relink files" all went wrong. It found photo's with te same name (but in fact other pictures). I deleted those. I went in the library (within Finder) to the Original Files folder, replaced the aliasses of the files pointing to a _Trash folder within the library. I thought the right pictures were back: no. I've dragged those back from iPhoto into the event, with the music also (from iTunes). There double now, but everything is good... except for the exclamation-mark!

Then I made a new library, dragged the project to the new library: copy used clips except the two options given (proxy en render files?). The new event also got a exclamation mark. I played the whole thing: no crashes, no beach-balls, no red missing clips signs, no yellow exclamation-mark in the Timeline Index.

In the original project I can now scrub little pieces (without spinning beach-balls) and I see the pictures on the timeline (in the preview) but with a yellow exclamation-mark in the index...? It cannot be found in the eventbrowser... how strange is that? 

Since the update to Final Cut Pro X 10.1.3 I cannot go back to a previous library. What do I do. I'm afraid if I continue to edit things will go south later on.

Info:
MacBook Pro (15-inch Early 2011), OS X Mavericks (10.9)