Found the problem.
Investigating the hiveserver2.log showed that rangerlogger failed to flush data to the db due to permission problem (Internal Exception: java.sql.SQLException: Access denied for user 'rangerlogger'@'<server>' (using password: YES)
Error Code: 1045)
After granting the permissions it flushed the data successfully.
Now i just need to figure out what caused the loss of permissions and how the hell it is related to the server reboot....
Thanks for all your help !