I think it could be a PHP setting on the server that shows "Notices, Warnings and Errors", instead of just "Errors". An administrator could have changed it, or maybe upgraded the PHP on the server.
For a developent site you would like to also see the warnings, but for a production server you only want to see the errors. The php.ini file on the server has these values available that could be set accordingly:
error_reporting = E_ERROR
display_errors = Off
Or maybe that one line could have been missing from the files that your ISP replaced from the backups.
Custom certificate (plugin)
Undefined index error
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