This, as far as I can tell from searching the forums and elsewhere, is identical to a long standing problem that has never been addressed by the developers. I don't hope to get resolution, as this issue has been neglected for years, but I thought I would try the forums before I start making angry phone calls.
As has been described elsewhere, I get the message "error steam read from workshop k_eresultfilenotfound" for any mods I subscribe to from the Steam workshop. I have a number of mods already included that seem unaffected, but at a certain point (perhaps after I purchased DLC for the game) I became unable to add newly subscribed content. I run on Windows. I have tried all the usual remedies.