Not going to try to translate all the German in there, but if your MediaInfo is failing then you got conflicts on multiple instances with one not being compatible and then that is the one used. Newer MediaPortal versions use a wrapper to avoid that, but it is still not a perfect solution. If you've ever installed a program that registered MediaInfo then that will still cause a conflict, and you will have to remove that version to stop the conflict.
Scan your entire system/registry for duplicate MediaInfo processors.
And you mention that it worked in 1.22, but that is because the new MediaPortal methods weren't in use then, but I'm on stable 1.25 and they are in place for me, and work fine.
Another option is to manually update the mediainfo.dll with the 32-bit DLL download from the MediaInfo group, but that still won't fix it if you got a conflict in your system.