I have installes Coppermine Photo Gallery v1.5.16 a few weeks ago and I no longer can login on my own computer, it says "welcome guest" but the control buttons do not appear. On my wifes computer and on my computer at work I do not have this problem.
Seems that there's something wrong with your browser. Maybe a security software manipulates your browser's cookies. Hard to say without further information, but as it works with other computers it seems to be no issue with your gallery.