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Welcome to the forums, olaf06!
Do you have a firewall and are the various Avast services allowed through the firewall?
Double click the "a" ball in the Task tray, select the Instant Messaging provider in the left pane of the opened user interface, click the Customize button near the top right and in the Provider Configuration section, and in the newly opened window make sure there is a checkmark in the box for Miranda.
I hope this helps you. If not, please post again.
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