Hello Stefan,
Were you able to reproduce that issue in RSRT? Does it behave the same on the ABAP and JAVA stacks?
Also, if you have user exit variables in those reports, I would strongly recommend you to do a syntax check on them as many standard data elements have changed in 7.40 (see Note 1879618 - Pre-upgrade measures for upgrade/update to 7.4 for details).
Hope it helps,
Sebastien Francal