Thanks Banjo, I did restore an old backup and I know when the data started dissapearing, it happened last Thursday and then again yesterday. Last thrusday it was only two teachesr and only two groups (44 records in total) but yesterday I had something like 12 different teachers and 15/20 groups (something like 400 records), plus some of the spring term reports that I had as read/only in ePortal are missing as well.
Teachers were not overwriting the reports to each other, they were only writing/updating their own reports.
The dataserver version is the same as ePortal service.
I will continue checking the logs and re-checking some older backups. Thanks.