OK. Here are another couple of things to try:
1. Go to *Home*/Library/Audio/Plug-Ins/Components
and move the contents to another folder somewhere convenient.
Do the same for this folder:
Now try starting Logic and opening those problem projects. Any luck?
*If step 1 didn't help, then quit Logic & replace the contents of those two folders.*
2. Quit Logic. Double-click on one of the problem projects and immediately keep the 'Control' key held down. This will start Logic without loading the core audio driver.
When you get the message, choose NOT to load the Core Audio driver.
If the Logic opens ok, then I'd recommend opening a new project and manually copy the parts from one to another. Save the new project as a new project.
When done, quit Logic and restart as normal and open the newly created project.
I'd really recommend backing up your projects on a daily/weekly basis. It's unusual but projects can become corrupted.
Let me know which steps worked for you.