I was able to recreate / simulate this exact problem/issue by doing this below. Using the PHP error_log() function incorrectly and doing something dumb/non-standard/not correct/hinky like this below. So what you need to do is search through any/all plugin code for any plugins that you have installed that might be writing to the php error log for debugging purposes. What you will be searching for is the PHP error_log() function used incorrectly somewhere in that plugin’s code.
The exact simulation / test that was done was this:
error_log('<div><h1><ul>Testing logging HTML to the php error log file since I am an idiot who does not know how to write/use code correctly</ul></h1></div>', 0, "blah", "blahn");
The Result logged in the PHP Error log file:
[01-Apr-2014 22:09:33 UTC] <div><h1><ul>Testing logging HTML to the php error log file since I am an idiot who does not know how to write/use code correctly</ul></h1></div>