To reproduce: create a folder with many textfiles where the sum of the textfiles' names is larger than 32768 (e.g a folder with 700 files each with a name longer than 50 characters). Select them all, right click, and choose "Edit in Notepad++". An error occurs.
When opening files using NppShell, it tries to create a single command-line call. But there is a limit to how long such a call can be (2048 or 32768 chars depending on Windows version). The files need to be opened in batches to avoid creating too long command-lines.