First of all: The new client looks fine and used to sync without any problems.
But today many of my libraries do not sync at all with the new client on Windows 8.1. The old clients (Linux CLI, Window 8) continue to work as expected so I don't think it's a server issue.
[10/09/13 12:16:52] sync-mgr.c(496): Repo 'REPO1' sync state transition from uploading to 'error': 'Error occured in upload.'.
[10/09/13 12:17:11] sync-mgr.c(425): Repo 'REPO2' sync state transition from 'initializing' to 'downloading'.
[10/09/13 12:17:12] transfer-mgr.c(501): Transfer repo 'cc0094dc': ('normal', 'init') --> ('normal', 'check')
[10/09/13 12:17:12] sync-mgr.c(425): Repo 'REPO3' sync state transition from 'initializing' to 'uploading'.
[10/09/13 12:17:12] sync-mgr.c(425): Repo 'REPO1' sync state transition from 'initializing' to 'uploading'.
[10/09/13 12:17:13] transfer-mgr.c(501): Transfer repo 'cc0094dc': ('normal', 'check') --> ('normal', 'commit')
[10/09/13 12:17:13] transfer-mgr.c(501): Transfer repo 'cc0094dc': ('normal', 'commit') --> ('normal', 'fs')
[10/09/13 12:17:17] transfer-mgr.c(501): Transfer repo 'cc0094dc': ('normal', 'fs') --> ('normal', 'get-chunk-server')
[10/09/13 12:17:17] transfer-mgr.c(501): Transfer repo '92e237ee': ('normal', 'init') --> ('normal', 'check')
[10/09/13 12:17:17] transfer-mgr.c(501): Transfer repo '8263ee42': ('normal', 'init') --> ('normal', 'check')
[10/09/13 12:17:17] transfer-mgr.c(501): Transfer repo 'cc0094dc': ('normal', 'get-chunk-server') --> ('normal', 'data')
[10/09/13 12:17:17] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:17] transfer-mgr.c(501): Transfer repo '92e237ee': ('normal', 'check') --> ('normal', 'commit')
[10/09/13 12:17:17] transfer-mgr.c(501): Transfer repo '8263ee42': ('normal', 'check') --> ('normal', 'commit')
[10/09/13 12:17:17] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:17] transfer-mgr.c(501): Transfer repo '92e237ee': ('normal', 'commit') --> ('normal', 'fs')
[10/09/13 12:17:17] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:17] transfer-mgr.c(531): Transfer repo 'cc0094dc': ('normal', 'data') --> ('error', 'finished'): Failed to download blocks.
[10/09/13 12:17:17] sync-mgr.c(496): Repo 'REPO2' sync state transition from downloading to 'error': 'Error occured in download.'.
[10/09/13 12:17:17] transfer-mgr.c(501): Transfer repo '8263ee42': ('normal', 'commit') --> ('normal', 'fs')
[10/09/13 12:17:19] transfer-mgr.c(501): Transfer repo '8263ee42': ('normal', 'fs') --> ('normal', 'check-blocks')
[10/09/13 12:17:21] transfer-mgr.c(501): Transfer repo '92e237ee': ('normal', 'fs') --> ('normal', 'check-blocks')
[10/09/13 12:17:21] transfer-mgr.c(501): Transfer repo '8263ee42': ('normal', 'check-blocks') --> ('normal', 'get-chunk-server')
[10/09/13 12:17:21] transfer-mgr.c(501): Transfer repo '92e237ee': ('normal', 'check-blocks') --> ('normal', 'get-chunk-server')
[10/09/13 12:17:21] transfer-mgr.c(501): Transfer repo '8263ee42': ('normal', 'get-chunk-server') --> ('normal', 'data')
[10/09/13 12:17:21] transfer-mgr.c(501): Transfer repo '92e237ee': ('normal', 'get-chunk-server') --> ('normal', 'data')
[10/09/13 12:17:21] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:21] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:21] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:21] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:21] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:21] transfer-mgr.c(531): Transfer repo '8263ee42': ('normal', 'data') --> ('error', 'finished'): Failed to upload blocks.
[10/09/13 12:17:21] sync-mgr.c(496): Repo 'REPO1' sync state transition from uploading to 'error': 'Error occured in upload.'.
[10/09/13 12:17:21] block-tx-client.c(709): Data connection is closed by the server.
[10/09/13 12:17:21] transfer-mgr.c(531): Transfer repo '92e237ee': ('normal', 'data') --> ('error', 'finished'): Failed to upload blocks.
[10/09/13 12:17:21] sync-mgr.c(496): Repo 'REPO3' sync state transition from uploading to 'error': 'Error occured in upload.'.
[10/09/13 12:17:17] ../common/peer.c(763): Delayed update from local-2(44fa1ab5), id is 1203, 500 Shutdown
[10/09/13 12:17:21] ../common/peer.c(763): Delayed update from local-2(44fa1ab5), id is 1212, 500 Shutdown
[10/09/13 12:17:21] ../common/peer.c(763): Delayed update from local-2(44fa1ab5), id is 1213, 500 Shutdown
[10/09/13 12:17:47] ../common/peer.c(763): Delayed update from local-2(44fa1ab5), id is 1228, 500 Shutdown
[10/09/13 12:17:51] ../common/peer.c(763): Delayed update from local-2(44fa1ab5), id is 1233, 500 Shutdown
[10/09/13 12:17:51] ../common/peer.c(763): Delayed update from local-2(44fa1ab5), id is 1234, 500 Shutdown