How should I do to fix syncing error?(Ver. 2.1.9)

Traceback (most recent call last):
File “aqt\”, line 382, in _sync
File “aqt\”, line 362, in _abortingSync
File “anki\”, line 112, in sync
File “anki\”, line 609, in applyChunk
File “anki\”, line 622, in _run
File “anki\”, line 557, in req
File “anki\”, line 496, in assertOk
Exception: Unknown response code: 400

How should I do to fix this error?(Ver. 2.1.9)

Please follow the steps on When problems occur - Frequently Asked Questions