Hi Christian,
I've faced the same issue for some time. Also tried all resolutions described above, even manually replacing the plugin for Rhino (from version 1.7.4. to 1.7.5), but nothing worked. Only difference is that I did not put -clean in my ini-file, but I added that to the shortcut (so my shortcut link is: "c:\Eclipse Luna 32bit\eclipse.exe" -clean). There is no difference between the 32bit and 64bit version either.
As a final resort, I removed my old Eclipse Luna installation completely, downloaded the latest Luna version and re-installed it. Then I ran the option "Install new Software", but I only selected SAP HANA Tools initially and that finally solved my issue (ofcourse only after restarting it with -clean option).
Hope this helps somewhat,
Gerwin