P2P DC++ filesharing app said: "could not open target file: the system cannot find the path specified"
when i tried to download file.

i think what was wrong was the configurable option in the Settings/Downloads called in my case "Unfinished downloads directory".
This directory have to exit and user need to have write access to it. If only some of the old download does nto work and new ones works. Maybe the old download destination directory does nto exist. In Queued items it may be changed or renamed.

I also have suspection (may be wrong) that the issue may be also related to too long path and downloaded filename (longer than 260 characters - some kind of Windows limit), so i set path like "D:" to minimize number of characters and it works. Note that path like C:\ may not work due it may be protected system path.