I encountered this already several times, while nothing was wrong. Sometimes going out of the theme editor, go back in then the isssue is solved.

I just had put in the code you see in this screenshot with the error notice. Clicked save, everything was fine. However, my curser ended after the } character and I wanted it to be on the next line, so I pushed my enter key. Then clicked save again, and then this error popped up every time I click save.

Same happening on my default theme. Is this a bug?

Based on your other topic, it sounds like you were getting logged out when you hit the save button.

Nope, this is happening multiple times, when I clicked OK I could just visit another part of the Admincp so I was not logged out.