Please mark sure you have unchecked the “Enable Protected View for Outlook Attachments” checkbox:
Open Word
Go to File | Options | Trust Center | Trust Center Settings | Protected View
Un-check the “Enable Protected View for Outlook Attachments” checkbox
In addition, please use the following code:
Restart-WebAppPool MsExchangeOwaAppPool
It will restart the Ms Exchange OwaAppPool.
If the problem persists, check ULS log for relevant error messages.
Below is the location, where you find the OWA logs.
C:\ProgramData\Microsoft\OfficeWebApps\Data\Logs\ULS
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