STR:
Manually save a session via File -> Save Session.
Open that session file in another editor and delete the first <NotepadPlus> line (-> file is corrupted).
Preferences -> Multi-Instance: select one of the non-default options.

Restart NPP.
Open some file.
Open the corrupted session file.

Result:
The "Session file is corrupted" message is not displayed.

Notepad++ v8.5.7.