[otrs] Requests for index.pl fail with error message "Illegal division by zero"

I activated and deactivated the Fontend::NotifyModule##5-Ticket:TicketEscalation module and now OTRS fails with the message Illegal division by zero at ../..//Kernel/System/Ticket.pm line 2103. I have got the system up and running again by commenting lines 2103 to 2107 in /Kernel/System/Ticket.pm which are: #my $Reached # = 100 - ( $WorkingTime / ( $Escalation{ $Map{$Key} . 'Time' } * 60 / 100 ) ); #if ( $Reached >= $Escalation{ $Map{$Key} . 'Notify' } ) { # $Data{ $Map{$Key} . 'TimeNotification' } = 1; #} Any ideas what's gone wrong and how to fix it without cutting out bits of the ticket module? Regards, Marc