After use of xlock in s15pup64 or ScPup64, I cannot open new windows in many cases. Only a restart of X seems to solve the problem. I read in some cases the error "unable to open display".
Bug in Xlock?
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Re: Bug in Xlock?
Found the bug:
https://bugs.archlinux.org/task/61704
I'm now using the slackware 15.0 package of xlockmore.