Roon arc offline downloads cannot be accessed?

There were several reports of downloads becoming corrupted or inaccessible in ARC offline mode, there seems to be a bug. E.g.,

Though this seems to be fixed at least in part. Make sure you have the lasted ARC update on the regular production update channel, ARC 1.0.8 (build 111). It may help to let your ARC resync on the local network.

If this does not help, resetting ARC or installing reinstalling it may be necessary. Yes, this will delete your downloads. I would give ARC a bit more time to stabilize and shake out the bugs before downloading 60GB to it.