SeaDrive stops syncing and giving unclear error

I have these errors constantly. What are they telling me?

[10/25/18 10:59:44] http-tx-mgr.c(911): libcurl failed to PUT https://domain/seafhttp/repo/512ff274-786a-4755-ab00-adef6a4371ea/block/a757c857403fc6bbca10f604a68e6928edf426b4: Operation was aborted by an application callback.

[10/25/18 10:58:43] http-tx-mgr.c(3655): Failed to get file 545cdfed8f33424b1de4b257f4c2ca74042e6bc6 in repo 512ff274.
[10/25/18 10:58:43] http-tx-mgr.c(1186): Transfer repo '512ff274': ('normal', 'data') --> ('error', 'finished')
[10/25/18 10:58:43] sync-mgr.c(604): Repo 'pdf-files' sync state transition from uploading to 'error': 'Error occured in upload.'.

Do you have a slow internet connection? Are your uploads/downloads taking a long time?

No. SeaDrive is installed on a AWS EC2 instance and Seafile Server is also on EC2. Internet is super fast. It just hangs like this.

What do the server logs show?

Nothing unusual. No error messages.

Nginx access log gives this

[28/Oct/2018:17:29:41 +0000] "PUT /seafhttp/repo/512ff274-786a-4755-ab00-adef6a4371ea/block/a757c857403fc6bbca10f604a68e6928edf426b4 HTTP/1.1" 400 0 "-" "Seafile/0.9.5 (Linux)"