Hi @willem,
Did you see my email about this issue from last Thursday? Thanks for posting here in any case!
This is a known issue if you create the query in Specify 7 but not in Specify 6 and try to use it as the basis for a new report. If you go to the query you are using to create the report and save it in Specify 6, then open it in iReport, it should show the correct list of fields and the field “string IDs” you see now should be resolved to the appropriate field captions.
I might need some more information to troubleshoot the exact cause here, so you can share the export of this report with me and I can take a closer look. Thank you!