I have not yet investigated the cause, but message upload is really slow.
On my laptop it fails on the first chunk. (After 30 seconds)
On my desktop, it succeeded, but took in the order of several minutes.
It might be the sleep, it may be server side issue, it may be something else.
It's not my network, I have 110Mbit/s upload, and have even checked with a VPN.
The chunk (3000 messages) appears to get sent instantaneously, but for some reason, the upload process is stuck idling after that, until timing out.