If getting this error -1: Conversion from string to type Boolean is not valid. [MergeCode-MailMerge] while sending group voucher via email in Group booking so perform below steps.
1) Close eZee on all machine except server machine.
2) Take DB back up from Server machine and close eZee on server machine
3) Keep tool Datadoctor_20171207 in application folder(C:\Program Files (x86)\eZee\eZee FrontDesk NextGen) which is attached below.
4) Start eZee Front Desk and check again.
