Its been a while since I did a fresh install so I figured I would give it a try. I did it on my Linux box and did not encounter the error that you mentioned, nor did the ajax mouse overs give me a problem. I usually run ost under IIS so I figured that it would be good practice to install it under s different OS. Line 151 of class.i18n.php reads: foreach (array_merge($dirs, $phars) as $f) {That tells me that its trying to load the language files to provide you with a choice of lang. Have you downloaded any of the language phar files and installed them? As far as the AJAX mouse over tooltop issue. please consult your Apache error log and let us know any issues you see in it.
