I thi9nk your problem is stemming from your previous attempt to install/use OpenSEO.
This is telling me that the Joomla! system is still trying to use the OpenSEO component, as that file (even that directory!) is not part of the "base" Joomla! install.
No, I believe they are setup with the installation of OpenSEO, and are not needed at all for the "built-in" SEF functionality
This is a "step backwards" in troubleshooting, and only complicates the issue(s) you will have to sort out.
My advice to sort this is as follows:
1) Return your Joomla! installation to it's "stock" codebase, including the .htaccess file.
2) TO do this you will probably need to uninstall any/all OpenSEO realted components/manbots/modules and make sure that you have all the original files components of the "core" distribution.
3) Once this is done, and you site is working correctly with no "sef stuff", you can test the "built-in" sef urls functionality per instructions in the Joomla! backend (check the option in the global config, and rename the htaccess.txt file to .htaccess).
4) Pay special attention to the .htaccess file at this point, becuase the one you have is very likely "borken." What is placed in theis file (uncommented, or commented out, and entries) is probably screwed up as a result of your different SEO/SEF attempts.
This process should "narrow the possibilities" for problems to occur, and make it much easier to sort out problems.
One last "friendly" comment about Joomla! and "search engine friendly/SEO" functionality:
Google, and all other major search engines, do not need any of this stuff to properly index your pages. Actually, the whole "SEF" part of "pretty urls" is of little practical use at present; page titles, content, and links are far more important.
That said, you might find that there is little need to install this stuff, and choose to avoid placing the extra load on the server required to rewrite urls.
I don't want to start a SEO debate here (there as many different informed opinions on that as there are ignorant ones!); I'm only sharing my experience with numerous Joomla! based sites. I have used the "built-in" SEF urls on occasion, and have yet to see it produce better search rankings than using "stock" Joomla! urls.