I maintain an article directory. After upgrading contact-form-7 to 2.4.1 I found myself locked out of my site at the user and administrator level. The error given was for the function is_multisite.
Access was regained by renaming the contact-form-7 folder in the plugins folder. Since wordpress could not find the folder it ignored the plugin calls and opened the site. Bless my luck stars.
Contact-form-7 is a nice useful plugin.