What to do when you receive a "Message Waiting" notification?#
When sending messages through the API the notification "Waiting for this message. This may take a while." is received instead of messages. When sending messages from the phone, messages are delivered normally.
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Solution to the problem:#
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Make sure you are using the latest version of WhatsApp on your phone.
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It is required to delete all sessions in the mobile device (section related devices) and scan the QR code again.