Ok, it's working now. The update and a restart did the job. 

Thanks for your efforts. 

That's strange, 

I will try it with 11.4.1 I'm still on 11.4. 

will post the result later.

Thanks for your fast replies. Your support is really great!

Hey Alex,

A friend of mine showed me that the document picker on an other app (Kodex) is able to open folders.

Thanks for your fast reply!
I tried several times but it never worked. I'm on iOS 11.4 by the way. Hopefully Apple changes this behavior.

It seems that the workaround isn't working anymore. There is no "select" field, only "cancel" is shown. On my IPad the select field is there and it works fine.