I am running WP 4.9.8.
I setup a new site in my WP multisite network recently and I can no longer activate or deactivate any plugins or themes at the site level nor can I network-activate or network-deactivate any plugins or themes at the network level.
When I try to activate or deactivate a plugin or theme at the site or network level, I get a white screen message in WordPress that states “the link has expired”.
What is also strange is when I move from any admin panel page to any other admin panel page, I can momentarily see a white screen with a single backtick (`) in the upper left corner of the screen for a second or two before the target admin page loads.
What I Know Existed At One Point In Time
I did happen to catch that my theme’s functions.php file had a backtick in the file at the very beginning of the file preceding the initial
Read more here:: Plugins and Themes cannot be activated or deactivated