My particular problem was with the (ancient, but useful) plugin bp-group-hierarchy. The manual fix recommended in the post BuddyPress 2.6.0 “Espejo” is now available (found here: https://github.com/ddean4040/BP-Group-Hierarchy/pull/21/files) got me back up and running. Thanks!
I am getting a fatal error after upgrade to BP 2.6.0 as well. Upgraded to WP 4.5.3 yesterday, then BP 2.6.0 today. WP_Debug reports ‘Call to undefined function bp_loggedin_user_id()’ in my custom child theme (Canvas parent theme) functions.php file. I tried the patch on bp-loader.php, r-a-y, but that was a no go for me. This is wiping out all front and back-end display on my local server. Fortunately, production is running WP 4.5.3 with BP 2.5.3, which is running fine. Still trying to debug…
