Hi Alexej,
Please provide more information like:
What is product version used?
Recent changes on server machine?
Was there any schedules running on date modified time?
Is any third party application/ Testing application used to schedule reports?
Is Platform search on continuous crawling?
If it is, please stop platform search and monitor for change.
Using query builder run a query like:
select si_update_ts from ci_infoobjects where si_id=<anyreportidhere>
Compare this date/time to what is displayed from CMC.
Hope this would help.
Regards,
Onkar