The command for that is
/viewlog.
Make sure you have enabled the use of commands:
Plus! > Preferences > Conversations > Main > Enable commands
The /viewlog command also takes an optional parameter
<email>. With this you can specifiy which contact you want to view the log from.
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You could have found this command yourself if you browsed the Plus! menus in the chat window...
Plus! > Send Command > Special Operations > Open Chat Log
Or by browsing the command helper window (type "/" in a conversation window)
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If you still want to use /chatlog as the command to open chat logs, then make an alias using QuickTexts:
Plus! > Preferences > Conversations > Quick Texts > Add...
Fast send chortcut: /chatlog
Message: /viewlog
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