Okay, sorry. Thus I just translate my last entry.
After testing around a few hours I figured out that the behavior seems not to be straight forward. Sometimes it's possible to search for files and for content if indexing is on. But most of the time only some files get found. Especially files that were opendend once will be in the content results list. But after a reboot the file search completeley fails.
My testing setup are thre content ident folders: one regular os x folder, one drive mounted by TrueCrypt (that's also uses OSXFUSE) and one BoxCryptor folder. For last one I tried new ones, existing ones, etc. pp. The TrueCrypt instead folder works perfectly, but there I have the DropBox syncing problem. The file must be unmounted before DropBox can sync it. The file encryption based way is my favorite.
Thanks for your help. I'm not sure where's the problem: BoxCryptor, OSXFUSE or my computer ;-)