Hi, hope I am not too late with this answer. I am certainly not an expert in any of this, but have taken enough advantage of these forums to feel the need to give back. I had exactly the same problem, and tried many different things, reinstalling the plugin, wordpress, the theme (THESIS 1.7), and still kept getting this error. I had a problem with thesis 1.7 after the upgrade with the custom file editor and the solution was to go that line, and add // to the front of it, to "escape" it, and it worked like a champ. That is a separate matter, but if you have the same problem, it works. Anyway, taking my cue from that, I read through the compatability(mispelled?, oh well) .php file on line 31. From what I could gather it had to do with backwards compatibility with earlier versions of WP. So, I experimentally added // to the front of line 31, thereby "escaping" it out, and voila! Everything works like it is supposed to.
I tried deactivating plug ins one by one, reinstalling WP, everything, and this simple little "cheat" worked. If anyone knows why, or how, or if it is bad or any of those negative things, let me know, but it is rocking like it is supposed to now! Yeah. Let me know if that helps!