Would it be possible that the user had something changed recently?
For example, his alias or UPN was changed.
And since we cannot get useful information from the error message, I would suggest using Teams Powershell to see if it can work as a workaround.
If not, if it would return more detailed information.
You can use the following cmdlet:
Add-TeamChannelUser -GroupID (Get-Team -DisplayName "Team name here").GroupID -DisplayName "Shared Channel name" -User "email@example.com"
If still no luck, please consider contacting support via opening a new service request in Microsoft 365 admin center>Support>New service request to have them look deeper into this issue.
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