I can't get sync with 6.6, nor the public version nor beta version. But the 6.5 has no problem with syncing. PS. I use both versions in China.
Evernote for Windows 6.6.4.5512 (305512) Public
OS: Windows 10.0.15063
User:
Memory: 88.2 MB
Created: 2017/08/03 02:02:16
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Bootstrap info: loadedFresh
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Unavailable
01:59:48 [23824] Utility Store request has failed. Error: Sync failed due to unexpected problem server side
01:59:49 [16948] 0% HTTP request failed, code: 500, error: Internal Server Error
01:59:52 [16948] 0% HTTP request failed, code: 500, error: Internal Server Error
01:59:54 [21484] Failed to retrieve messages from the Communication Engine Client
01:59:54 [21484] * HTTP request failed, code: 503, error: Service Unavailable
01:59:55 [3032] Client synchronization started
01:59:55 [25876] 0% Loaded updateCount: 34070
01:59:56 [25876] 0% HTTP request failed, code: 500, error: Internal Server Error
01:59:56 [3032] Client synchronization finished, status: failed
01:59:56 [3032] * error: Sync failed due to unexpected problem server side
01:59:56 [3032] * elapsed time: 1s
01:59:57 [3032] Client synchronization started
01:59:57 [25876] 0% Loaded updateCount: 34070
01:59:57 [16948] 0% HTTP request failed, code: 500, error: Internal Server Error
01:59:57 [25876] 0% HTTP request failed, code: 500, error: Internal Server Error
01:59:57 [3032] Client synchronization finished, status: failed
01:59:57 [3032] * error: Sync failed due to unexpected problem server side
02:00:05 [4188] Registered session count: 1, last session: 2017/08/03 02:00:00
02:00:09 [16948] 0% HTTP request failed, code: 500, error: Internal Server Error
02:00:33 [16948] 0% HTTP request failed, code: 500, error: Internal Server Error
02:01:19 [16948] 0% HTTP request failed, code: 500, error: Internal Server Error
02:01:47 [21900] Received status code 200 when accessing URL https://update.evernote.com/public/ENWin6/ReleaseNotes_6.6.4.5512_en-us.html