Can confirm that as of yet this is not fixed on 3.4.0. It just seems to get stuck on certain specific (usually small?) files. Pausing and re-starting sync gets it uploading other files again, but once it has found those specific files again and fills up its queue with them uploading stalls with a full queue full of files that are stuck at 0 bytes.
I encountered the same thing a few months ago when uploading a lot of files. My experience then was that if you just leave it running in background for a very long time (i.e. several days) it will eventually proceed.
The way I have to get it unstuck is to move folders/files it gets stuck on out of my InSync folder, quit InSync, start InSync back up, allow it to fully sync (doing the same dance again with any other files it gets stuck on, so usually I have to repeat this thing 4-5 times before it runs out of files to get stuck on), and then move all of those files back into InSync’s folder where it does then sync them correctly. The fact that moving files out and back in “fixes” it would indicate this is not a problem with the files themselves.