Make sure that you have set the focus to the "loginid" textbox at the time of page load. The deafult button responds to the form and thus it may need the focus to be on any of the control inside that form which seems to be browser specifi behavior, although I am not sure about this.
To set focus on a textbox, in page load event you may simply write-
textbox1.Text.focus();
If this doesn't work, try this-
ScriptManager.RegisterStartupScript(this, this.GetType(), "Focus", "$get('" + textbox1.ClientID + "').focus();", true);
or
ClientScript.RegisterStartupScript(this, this.GetType(), "Focus", "$get('" + textbox1.ClientID + "').focus();", true);
I hope this works. In case it doesn't work, please let me know.