For me, I identified this when installing a site on my local wamp, from a backup. It's worth mentioning that the Joomla version is 3.6.X
The fix to this does involve a hack to core files, which I normally would not recommend, but I am yet to find a better solution. Writing to the core files hasn't been a deal breaker for me in this instance, as this problem has only been noticed on my local wamp and not on a live site, and I haven't had the need to push my hack up to the live site - So for the moment, making a hack to my local version has solved the problem.
As yet, I haven't seen if the problem still exists once the version of Joomla has been updated - Nor can I say with any certainty that this issue only exists with Joomla 3.6.X (but that has been my experience).
Ok - On with the fix. Remove the lines on code in red, and insert the lines in green.
plugins/system/t3/includes/depend/t3filelist.php
| // update path to this template | ||
| $path = (string) $this->element['directory']; | ||
| + $path = JPath::clean($path); | ||
| $options = array(); | ||
| // get files in template path | ||
| $this->directory = $this->element['directory'] = T3_TEMPLATE_PATH . DIRECTORY_SEPARATOR . $path; |
plugins/system/t3/includes/depend/t3folderlist.php
| $paths = T3Path::getAllPath ($path); | ||
| foreach ($paths as $path) { | ||
| - $this->directory = $this->element['directory'] = $path; | ||
| + $this->directory = $this->element['directory'] = JPath::clean($path); | ||
| $tmps = parent::getOptions(); | ||
| foreach ($tmps as $tmp) { | ||
| if (in_array($tmp->value, $vals)) continue; |