Please, update theme to the version 3.12. It’s available on ThemeForest.
Do the backup of theme options before the update (Theme Options > Import/Export > Options > download .json file)
After update disable XStore Core plugin, re-activate theme, then activate XStore core plugin and go to Appearance > Install plugins > update Core plugin.
We have made some improvements for the cart/search/wishlist options in the header http://8theme.com/demo/xstore/update-history/ so, after update very likely you will need to choose the position of these options in theme settings.
Let me know if after that problem with brands still persists and provide admin panel and FTP access.