It seems that running xss-lock as `xss-lock --session=${XDG_SESSION_ID}` works around this problem.
- Bug#1040917: xss-lock: Crashes after unlock Nikolaus Rath
- Bug#1040917: Acknowledgement (xss-lock: Crashes after u... Nikolaus Rath
- Bug#1040917: Acknowledgement (xss-lock: Crashes aft... Ian Campbell
- Bug#1040917: Acknowledgement (xss-lock: Crashes... Nikolaus Rath
- Bug#1040917: Acknowledgement (xss-lock: Cra... Ian Campbell
- Bug#1040917: Acknowledgement (xss-lock... Nikolaus Rath