Problems with German umlaut in RNotebooks

I'd like to use RNotebooks with my students but it seems that the solution "File -> Reopen with Encoding" working fine for RMarkdown-Documents is broken with RNotebooks. It seems that the bug isn't new (I had same issues when teaching some months ago) and I also tried the newest build.

Does anyone know a solution for this problem? As it is now I simply cannot use RNotebooks with students starting to learn R.