After certain time data_engine get disconnected from database server.
you can get at data_engine log messages like:
Mar 3 11:15:31:776  de: [Monitor] has disconnected from database
Customer was using SQL 2016 with database in compatibility mode SQL Server 2012.
MS SQLServer 2016,is not supported with UIM 8.51
For checking supported databases review Compatibility Matrix at:
CA_UIM Database on SQL Sever 2016 using SQL Server 2012(110) Compatibility level
Change database to use MS SQL Server 2016 in 2014 Compatibility mode or Upgrade to UIM 9.02