Hi Paul Roos,
Welcome to our forum!
Please refer to the link and have a try: Chat with others in Microsoft Teams - Microsoft Support
If this doesn’t work, please provide more information for our better research:
- Does the issue occur recently in Teams Desktop Client?
- Did you have any specific operation before the issue occurred?
- Could you please provide a screenshot? (To protect your privacy, please be careful to hide your personal information when uploading.)
Hope it helps.
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