>>>This server has been in use since 2008
>>>Suddenly today, rows disappear.
>>>It might be a coincidence, but the rows seem to be disappearing after a backup occurs.
>>>Many backups have been made before with no issues.
>>>Any ideas?
>>
>>The question is unclear. Did you find some table(s) missing rows? If yes, how did you find it? Is it possible that these rows were deleted? Do you have any audit in place?
>
>Run a report
>Rows are there
>Back up the system
>Run a report
>Rows are gone
>No audit

1. Examine the backup job

2. In SSMS run the query that drives the report. Perhaps some parameters have changed.