I have pretty the same problem right now. I loaded about a hundred mods into the queue within about 20 minutes, only to find out that the download had stuck after the third mod. Pressing the pause buttons didn't help either and so I restarted Vortex. In the hope that the downloads would start again. But unfortunately none of the downloads started and by clicking on the resume buttons I got only the mentioned error message except for 2 or 3 downloads. Thats pretty frustrating.