I received this message on 4 Joomla! builds after logging off last night and logging back on this today. It appears immiediately after logon. The admin menu appears but the control panel does not. The Retun to Control Panel button does not load the cpanel. Most menu options on the menu work fine with the exception of the Control Panel option and clicking the Joomla icon in the upper left corner. Note that I upgraded to 3.8.1 immediately after the release notification appeared and continued to work on the four builds for 16-18 hours or so after the upgrade. I made one K2 update after the 3.8.1 upgrade. K2 launches from the Component menu fine. Initially, I thought the K2 upgrade may have made a change to its Stats and Icons modules, but the control panel does not load when those modules are unpublished. I flushed the browser and Joomla caches and deleted all session files. The problem repeats on both Chrome and Firefox. I can use the backend fine--just can't access the cpanel.

I'm running Windows 7 with Xampp (PHP 7.1.6 and MariaDB 10.1.24). The 3.7.5 to 3.8 upgrade went well without a hitch. All four builds are running as virtual hosts and have been since 3.7.5. Debug is turned on. The four sites use different combinations of components, but the common components are CB, K2, Akeeba Backup, ACYMailing, AlterBrains Bridge, AlterBrains Glossary, JEvents, IgniteGallery, and FeedGator 4.0 (nivong/feedgator). None of these components worked fine after the 3.8 upgrade and all have been accessed muiltiple times since 3.8.