[Plugin: Events Organiser] Categories messed up

I have this plugin installed and it works fine off it sown template but in the theme template it appears not to get the categories…these have also been terrible problems, having to delete them and start again.

I get an error in the themes functions.php which is directly related to categories:

Catchable fatal error: Object of class WP_Error could not be converted to string in /home/xxxxx/public_html/wp-content/themes/THEME/functions/theme-functions.php on line 406

is there a way round this? What do i need to do as the inbuilt templates do not work. Or should I just edit these templates