See linked bug. From https://bbs.archlinux.org/viewtopic.php?pid=1070632 , some other X programs may also have problems (like xkill) I can confirm I had to clean up 12GB of .xsession-errors filled with the select line (my backup cron started to complain about the file size change)
To reproduce, screensavers that must load images from a user-defined location (GLSlideshow, photopile, etc..) work "best". Apparently the locking part of xscreensaver is unaffected, it's just the display part, but may be worth a check
Fixed in 1.1.3.