Hello,
i had this several times now:
Dragging from the Files App to DTTG in Split View or Slide Over does not work.
Instead DTTG issues an error.
This happened with regular PDFs, PDFs created from Websites and with epub files.
Sometimes, it works when I repeat the drag-and-drop several times, but it is not reliable.
I will attach an screenshot where DTTG is to the right as Slide Over.
I am on iPadOS 13.5 and the latest DTTG.
