I would like to ask following questions to get more information.
What is the detail of error when the user failed to change password?
Does the issue affect only specific users? Or all the users in your organization?
I would suggest you logon to EAC with an Exchange Admin account, select Server/virtual directories.
From within the list of Exchange Virtual Directories, select OWA (default website).Open its Properties, which by default looks like the image shown below.
Check the “Change password” option, ensure it has been selected.
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