Because my set up is different, I don’t know if this will help or not, but I upgraded to the latest Sonarr V3 beta from V2 which had been working fine and immediately started getting the same error as you indicated. (Sonarr and SABnzbd all running on Windows 10.)
Import failed, path does not exist or is not accessible by Sonarr: C:\Users\Chris\Downloads\complete\Better.Things.S03E12.1080p.WEB.H264-METCON. Ensure the path exists and the user running Sonarr has the correct permissions to access this file/folder
Sonarr would detect my episodes just fine and send them to SABnzbd to download, but then they would just sit there in my Downloads\complete folder, and would not rename and then move to the proper location. Checking the logs showed the same error as you. I went into the Task Manager --> Services, and then right clicked on the Sonarr service and selected “Go to Details”. It showed that the User Name for Sonarr.Console.exe service was “LOCAL SERVICE”…so I right clicked on Sonarr.Console.exe, chose Properties–>Security–>Advanced–>Add…then clicked on the “Select a Principal” link, typed in “LOCAL” then clicked the Check Names button, and selected the “LOCAL SERVICE” name. I gave it full control permissions, and applied the changes. I then right clicked on my C:\Users\Chris\Downloads\complete folder (where the episode folders & files sit after downloading before being renamed & moved by Sonarr) and added the LOCAL SERVICE user to this folders permissions as well giving it “Full Control”. I then restarted the service and then opened Sonarr. The DownloadEpisodesImportService immediately started working and renamed and moved my episodes that had been sitting there and failing all night. Before doing this, I had gone through and tried to open up the permissions on the folders but nothing seemed to work until I did the above and added the LOCAL SERVICE user to the mix. Not sure if this will help anyone but I’m glad it’s finally working.