First, install the current customer version of RFEM 5 / RSTAB 8.
If the problem persists, the software is most likely blocked by an antivirus program. We recommend adding Dlubal Software programs, including the working directory, to the exceptions of all existing antivirus programs.
You can find the path of the working directory in the menu "Options" → "Program Options", tab "Data Files" → category "Working Directory".