This is a TeX error (\verb is used to display symbols verbatim). For jsMath the most likely problem is that your jsMath code in WeBWorK needs to updated to the latest jsMath version. I do remember that the ability to handle \verb was added to jsMath some time ago, but I don't remember exactly when.
I suspect the newer versions of str_cmp use \verb to better display the answer entered by the student -- for example ordinary text won't be italicized, spaces are preserved and so forth.
I'm not sure why "image" mode using dvipng would not work for you. The texpreamble file in webwork2/conf/snippets should pull in TeX code that can process \verb. One thing to remember is that the images are cached -- so if there is a mistake and you think you have fixed it you will need to clear the equations cache to make sure that you see the fix.
I tried your problem on hosted2.webwork.rochester.edu and the preview correctly handled \verb in both the image and jsMath preview.
There are maa10x courses at http://hosted2.webwork.rochester.edu/webwork2 where you can test this problem yourself -- use profa/profa as login and password.
Hope this helps with the trouble shooting.