Hi Fabian,
A kernel upgrade seems to be the solution. Please read SAP note 2197569: ITS HTML Viewer: documents with sapr3 url not shown. The correction is available as of kernel 7.21 patch level 523. I would recommend the adoption of the stack kernel (SAPEXE/SAPEXEDB) patch level 600 + disp+work patch level 610 (or higher).
Kind regards,
Cris