For XnView 1.80.1, this has unfortunately changed for some reason. BACKSPACE still moves backwards (as does PAGE UP), but Space Bar no longer moves forward. Instead Space Bar is used to start a slide show (?!). The new key assignment (and only key I can find) to move forward through a series of images is the far more inconvenient PAGE DOWN key.

Yes, there is an option for the mouse wheel to move backward and forward through an image series, but I use that for zooming. It is no replacement, especially since I've been using the keyboard for many years to handle image series progression. This change is so inconvenient here that I'm very likely ditching XnView and going back to ACDSee. Please (please, please) put the Space Bar back to "next image". All other image programs I've used and like do it this way (ACDSee, Irfanview, SlowView, etc.). If the dev doesn't want to do this, then how about giving us an option (far more complex to code) to reassign keys?
XnView became my favorite image viewer a while back and I used to recommend it to all my friends & associates. This change has stopped me from doing so. Hopefully it can be reconsidered.