I get this error out of the blue, when trying to login to the CP, possibly because the ISP made some updates to PHP/MYSQL.
An Error Was Encountered Unable to load the requested file: account/login.php
EE is version 2.1.6 - an older customer site, has been stable for a number of years, has lots of new entries every day. No dev work going on.
Customer can't go to a later EE version until later this year. Rolling backups are taken each working day and kept for five days. The earliest backup in the current series also throws the error, which mmeans that last successful superadmin login was more than five days ago.
The front end appears to be unaffected according to the customer.
ANy pointers as to where to start looking appreciated. Similar report affecting EE 2.7.2, made on Ellislab forum in February has no replies.