Just upgraded a site from 2.5.5 to 2.8.1 with the plans of adding MSM once 2.8.1 was installed. I successfully upgraded to 2.8.1 and when I go to turn on MSM it kicks me out of the CP with the following error:
An Error Was Encountered
Site Error: Unable to Load Site Preferences; No Preferences Found
When I set $config['multiple_sites_enabled'] = "n"; I'm able to log back in with no errors. Not sure where to start troubleshooting?
Figured it out! In my config.php I had dynamically set config variables with PHP variables:
$base_path = $_SERVER['DOCUMENT_ROOT'];
$config['tmpl_file_basepath'] = $base_path.'/assets/templates/';
which was causing connection issues with the database with MSM enabled. Not exactly sure how MSM works, but explicitly setting these variables for each domain fixed the issue along with uncommenting & setting the MSM $assign_to_config variables in /index.php & /system/index.php.