Server stops backup and hands

Hey,
i am using Urbackup for 1 month now and i am encountering some problems since two days now.

I have a Ubuntu server where Urbackup Server is running and three Windows Clients to be backuped. The Backupdir of Urbackup-Server lies within a Veracrypt Volume.

Right now the Server runs in Debug mode and while trying to do a full backup of my Windows 10 PC it freezes around 93%. The Urbackup Server can not be stopped as it remains as a zombie process.
The Logfile shows this errors:

2016-02-25 08:54:47: WARNING: Restarting shadow copy of E:\ because it was started by this server
2016-02-25 09:07:13: WARNING: Failed to write to file... waiting...
2016-02-25 09:07:26: WARNING: Failed to write to file... waiting...
2016-02-25 09:08:19: WARNING: Failed to write to file... waiting...
2016-02-25 09:08:37: WARNING: Failed to write to file... waiting...
2016-02-25 09:09:24: WARNING: Failed to write to file... waiting...
2016-02-25 09:09:43: WARNING: Failed to write to file... waiting...
2016-02-25 09:09:54: WARNING: Failed to write to file... waiting...
2016-02-25 09:10:06: WARNING: Failed to write to file... waiting...
2016-02-25 09:10:16: WARNING: Failed to write to file... waiting...
2016-02-25 09:10:43: WARNING: Failed to write to file... waiting...
2016-02-25 09:11:20: WARNING: Failed to write to file... waiting...
2016-02-25 09:11:35: WARNING: Failed to write to file... waiting...
2016-02-25 09:11:49: WARNING: Failed to write to file... waiting...
2016-02-25 09:12:05: WARNING: Failed to write to file... waiting...
2016-02-25 09:12:31: WARNING: Failed to write to file... waiting...
2016-02-25 09:13:24: WARNING: Failed to write to file... waiting...
2016-02-25 09:13:58: WARNING: Failed to write to file... waiting...
2016-02-25 09:14:15: WARNING: Failed to write to file... waiting...
2016-02-25 09:16:08: WARNING: Failed to write to file... waiting...
2016-02-25 09:28:45: WARNING: Failed to write to file... waiting...
2016-02-25 09:29:13: WARNING: Failed to write to file... waiting...
2016-02-25 09:29:33: WARNING: Failed to write to file... waiting...
2016-02-25 09:29:47: WARNING: Failed to write to file... waiting...
2016-02-25 09:30:19: WARNING: Failed to write to file... waiting...
2016-02-25 09:30:37: WARNING: Failed to write to file... waiting...
2016-02-25 09:31:51: WARNING: Failed to write to file... waiting...
2016-02-25 09:32:20: WARNING: Failed to write to file... waiting...
2016-02-25 09:32:42: WARNING: Failed to write to file... waiting...
2016-02-25 09:32:52: WARNING: Failed to write to file... waiting...
2016-02-25 09:33:13: WARNING: Failed to write to file... waiting...
2016-02-25 09:33:40: WARNING: Failed to write to file... waiting...
2016-02-25 09:33:52: WARNING: Failed to write to file... waiting...
2016-02-25 09:34:03: WARNING: Failed to write to file... waiting...
2016-02-25 09:34:16: WARNING: Failed to write to file... waiting...
2016-02-25 09:34:41: WARNING: Failed to write to file... waiting...
2016-02-25 09:34:56: WARNING: Failed to write to file... waiting...
2016-02-25 09:35:12: WARNING: Failed to write to file... waiting...
2016-02-25 09:35:22: WARNING: Failed to write to file... waiting...
2016-02-25 09:35:33: WARNING: Failed to write to file... waiting...
2016-02-25 09:35:43: WARNING: Failed to write to file... waiting...
2016-02-25 09:35:53: WARNING: Failed to write to file... waiting...
2016-02-25 09:36:03: WARNING: Failed to write to file... waiting...
2016-02-25 09:36:13: WARNING: Failed to write to file... waiting...
2016-02-25 09:36:23: WARNING: Failed to write to file... waiting...
2016-02-25 09:36:33: WARNING: Failed to write to file... waiting...
2016-02-25 09:36:43: WARNING: Failed to write to file... waiting...
2016-02-25 09:36:53: WARNING: Failed to write to file... waiting...
2016-02-25 09:37:03: WARNING: Failed to write to file... waiting...
2016-02-25 09:37:14: WARNING: Failed to write to file... waiting...
2016-02-25 09:37:24: WARNING: Failed to write to file... waiting...
2016-02-25 09:37:34: WARNING: Failed to write to file... waiting...
2016-02-25 09:37:44: WARNING: Failed to write to file... waiting...
2016-02-25 09:37:54: WARNING: Failed to write to file... waiting...
2016-02-25 09:38:04: WARNING: Failed to write to file.2016-02-25 09:38:04: WARNING: Failed to write to file... waiting...

Until now i was hoping to find a good and easy backup solution for my home network but at the moment i do not see a stable and working server. Any ideas what the problem might be?

Sincerely yours
Markus

I know @uroni will ask, what version of client and server are you running?

Sorry i forget that. My Server Version is 1.4.13.1373-1ubuntu1~trusty and my clients all use the latest client 1.4.11.

The error occurs if it cannot write to backup storage (or a temporary file if you enabled temporary file buffers). It retries infinitely while freeing space if free space is low.

You should check if there are issues with writing to backup storage e.g. via dmesg. That you cannot stop the server further indicates it it stuck in kernel space (probably while writing).