This error is just telling you what you already know, that the list item isn't accessible by the workflow, meaning it's either been deleted or had the permissions changed so the workflow can't "see" it.
The obvious place to look are any steps in this workflow OR any other workflows or trggered actions that could be acting on this list that either delete anything (even if you don't think it should be your list item) or change permissions.
The one thing I will hightlight is that the Sharepoint + Nintex environment is chronically prone to timing issues (so-called "race conditions"), which means that if your workflow(s) make any assumptions about the sequence order in which certain actions will be executed, that's where you need to look.
Unfortunately the very act of adding "Log to Workflow History" debugging steps to your workflows can itself change the timings, snd therefore either introduce or mask bugs.
Pllain lists are only moderately tricky in this respect because the lifecycle of a basic list item is fairly simple and rational, so if the issue arises it will be from within the workflow logic. By comparison the lifecycle of a document library item is nightmare, full of bizarre turns and with at least one special case that a workflow simply can't deal with.
I haven't done much with discussion lists, but like much in SP they follow the "everything is a list, but some lists are so hacked ithey are barely recognisable" fomat. You have parent/child relationships between items (thread starts posts vs. replies) and the edit form for a discussion is locked and unchangeable, so form-based workarounds to deal with lifecycle problems aren't available to you.
Try to reduce your problem to a minimal test case of a few actions that either works or doesn't. If it does, gradually re-introduce the structure of the real workflow until it breaks. I can't say what might be causing your problem, but I will warn you that the out-of-order execution thing can really bake your noodle when it comes to trying to understand workflow errors.
Good luck: Colin