this is a bug As a workaround open the \App_Config\Include\Sitecore.Speak.config file and comment out the following line:
<override dialogUrl="/sitecore/shell/Applications/Dialogs/Internal%20link.aspx" with="/sitecore/client/sitecore/common/dialogs/InsertLinkViaTreeDialog" />
After that Sitecore will use standard dialog instead of the Speak dialog.