I have a temporary solution that worked for me. Find your file includes\init_includes\overrides\init_templates.php. On that file find the following code on line 41:

PHP Code:
 $template_dir 'classic'
   if(isset(
$_GET['template_switch_id'])){
    
zen_setcookie("zctemplate"$_GET['template_switch_id'], time()+(60*60*24*365));
    
$template_dir $_GET['template_switch_id'];
  } else {
      if(isset(
$_COOKIE["zctemplate"])){
        
$template_dir $_COOKIE["zctemplate"];
        
zen_setcookie("zctemplate"$_COOKIE["zctemplate"], time()+(60*60*24*365)); 
      }
  } 
and change it to:
PHP Code:
 $template_dir 'classic'
   if(isset(
$_GET['template_switch_id'])){
    
zen_setcookie("zctemplate"$_GET['template_switch_id'], time()+(60*60*24*365));
    
$template_dir $_GET['template_switch_id'];
  } 
the only problem with this solution is that the chosen template will not be stored in a cookie for future visits.

I have this addon installed in one of my websites and works fine. I installed the same files on a different website and ran into the same problem you did.

If I find a better solution I'll post it here.