I just hit this wall too and YOUR post was the ONLY thing even remotely close to my situation BUT i am using windows 10.  The ONLY thing i changed since last server run was added the rustedit dll. literally just a fresh download 2 weeks after my initial download I tried redownloading the rustedit dll when i was having probs working in rustedit [figured it - i was using old ver of rustedit]. - NOW for some reason NONE of my plugins will load today. Literally just stoped working today for no good reason. Just says plugin not found! LOL. I have learned MUCH on my own just researching and testing - but a complete fail to load ANY plugins SHOULD mean either oxide or rust needs some love - it just worked yesterday - i mean wth... XD

Found this: "In nearly all cases where plugins are not working, is after a Steam update." - dint see a steam update today, but I will report back on what happens.
Currently removed all plugins and start server. THen i will add all plugins and see if they load - i left the config files THIS pass.

FINAL EDIT: Plugin work was a waste of time. Discovered there was an Oxide update 8 hours ago.

Viola! Steam update or Oxide update. In this case it was an Oxide update. CASE SOLVED.

:/ i guess i will quit writing short stories on the net and just do what has been working best so far... -->research the net and use a lil brain power [no intent for OP @all, just me... ALLLL me]...

PEACE!!!