I am here because I received the error as well, but I seemed to have fixed it from my end. Hopefully it will work for you too.
When I’m not maintaining my sites, I enable “Remove File Writing Permissions.” Part of my conflict is that I also use W3 Total Cache, but here is what I did.
1. Deactivate iThemes Security.
2. Empty all the page caches from within W3 Total Cache.
3. Reactivate iThemes Security.
4. Test by enabling/disabling “Remove File Writing Permissions.”
Good luck.