Q: Install El Capitan

Hello, valbogatan.  

Thank you for visiting Apple Support Communities.  

I understand you are receiving an error after restart when you installed El Capitan.  Since you have gone through the different start up methods, I would try repairing your disk.  Here are the steps to access Disk Utility via the internet recovery option.  

Try Disk Utility

Use these steps to use the Disk Utility app to verify or repair a disk.

1. If you're using OS X Lion or later, start the computer from Recovery System or Internet Recovery. If you're using an older version of OS X and your computer came with a Mac OS X Install disc, insert the installation disc and restart the computer while holding the C key instead.
2. When your computer finishes starting up, choose Disk Utility from the Utilities window, or from the Installer menu if you're started from an installation disc.
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Startup Disk (usually named "Macintosh HD").
6. Click Repair. Disk Utility checks and repairs the disk.

Tip: With Mac OS X v10.6 and earlier, always start up your computer from an Install or Restore disc when using Disk Utility to verify or repair your startup volume. Otherwise, you might see some disk error messages.

 

Resolve startup issues and perform disk maintenance with Disk Utility and fsck

After this is done try reinstalling El Capitan and let me know if the issue persists.  

Cheers