I would recommend opening a support ticket to look into further
I have had the same issue many times in the past which only came up after upgrading from 8.5 to 9.01. We are currently running into the same issue with 9.06 after the we were told to upgrade to resolve the issue.
I have found it only happens at random times, and is usually caused by having a large number of specific monitors as a reports data source. When the issue happened for me, the data source was completely removed from the report template, and I had to set it all up again.
Since changing the report from using specific monitors to using smart groups, the issue has not returned. It has been around 2 months without an issue since making this change.