Well I am unable to reproduce this issue. I used the config described by dr29bart and notanother with 1.7 and the files show up as expected in the report – see screenshots. I am also attaching the config.xml of the project I have locally, in case this rings a bell to someone.
dr29bart notanother would you mind telling us whether the files exist on your workspace ? For instance dr29bart, do $WORKSPACE/target/html-reports/report.html and $WORKSPACE/target/html-reports/results.txt exist ?
Also can you confirm that upgrading to 1.8 resolves the problem ?
If you have the config.xml of your projects handy, this might also be helpful.
Also getting this in our Jenkins system after the update to 1.7.

Here is the configuration that was working prior to the update: