You problem is most likely compatibility issues with your theme Arras.
As of April 21, 2016 Arras is no longer in active development and support is limited.
If you’d like to leave a note about an Arras issue, the best place to do it is on the Arras Issues page at Github. https://github.com/iCaspar/arras/issues
Try switching your theme to one of the default twenty* themes to see if the problem persists.
Thread Starter
Tim
(@tdranger)
After activating Twenty Sixteen, same problem. I cannot customize even twenty* themes.
I suggest reinstalling wordpress.