Did you have a previous fop2 version installed? The error means that the html/js files were not installed properly, of that you have a mixed bag of cats, probably mixing files from different versions. In the Elastix version for example, there is a fop2.html file that is not part of the standard fop2 distribution (but exclusive to elastix), when you click on a fop2 link in the elastix interface, it will load that file. Now, if you install 2.28 over, you have to log via index.html and not fop2.html, otherwise the mixed versions will come up. One way to overcome that issue is to copy index.html over fop2.html, like

cp /var/www/html/fop2/index.html /var/www/html/fop2/fop2.html

I am not sure if that is the case as I do not have enough information, but it could be.

Also, if you install fop2 from market OVER the 2.28 version installed from tarball, you will probably have similar issues.