As have other users, I too have been experiencing the following error while testing cache in Mod Rewrite mode:
Fatal error: Cannot use object of type WP_Error as array in /home/mysite/public_html/wp-content/plugins/wp-super-cache/wp-cache.php on line 977
The pages also fail to be fetched while testing cache.
While testing & clearing cache I also received the following error:
Fatal error: Out of memory (allocated 35127296) (tried to allocate 103396 bytes) in /home/mysite/public_html/wp-includes/class-http.php on line 1106
Also noticed that the "clear cache" button doesn't always clear all cache, as WPCached files appear after clearing the cache & can only be cleared individually by expanding the cached files window in the Contents tab.
Tried Legacy mode & the time stamps don't match, although the pages could be fetched. PHP mode seems to be working...for now. I say for now because the Mod Rewrite mode again began working only to fail shortly thereafter. There was a point when the entire site went down for a few moments, casting the following error:
"Unable to establish a database connection."
Could there be issues with the recent WP upgrade?