Method 1
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1) Start ARMA3
2) Select Play
3) Chose Muliplayer
[Some times it will happen when loading the servers] if not, continue
4) Select and Join a Server
Method 2
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1) Start ARMA3
2) Select Play
3) Chose Scenario
4) Chose a Mission
5) It will Freeze [This will sometimes warrant the error message or require the task to be ended via Command Prompt]