One error that was reported was that when the "Notify Me" link was clicked for an item had been checked out by someone, this
error message came
An error occurred during the processing of
/library/Lists/Transactions/NotifyMe.aspx. The resource object with key
'NotifyMe' was not found.
The solution found by the poster was to do both iisrestart and to restart the "Windows SharePoint Services Web Application" from within the Central Administration site (Operations\Services on Server).
Thanks to "DangerousDan" for question and answer!