It's hard to say what's causing it, but it could be some security software or a recent Word update. Usually when there's a generic bug in Word or Zotero we get many reports about it, and we've had some Chinese users have issues when using Word in Chinese that got resolved when switching to English.
Can you create a new Windows user account on this machine and check if citing is broken there in the same way? If it is can you create a new document and produce a minimal case where citing breaks? Make sure it is still broken after restarting Word. Then send it to support@zotero.org with a link to this thread.
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Can you create a new Windows user account on this machine and check if citing is broken there in the same way? If it is can you create a new document and produce a minimal case where citing breaks? Make sure it is still broken after restarting Word. Then send it to support@zotero.org with a link to this thread.