This is now broken for quite a long time: Every change of the brightness slider makes the power manager plugin crashing: From the syslog: Mar 08 18:37:39 [pkexec] massimo: Executing command [USER=root] [TTY=unknown] [CWD=/home.crypt/massimo] [COMMAND=/usr/sbin/xfpm-power-backlight-helper --set-brightness 8] Mar 08 18:37:40 [kernel] panel-5-power-m[14500]: segfault at 0 ip 00007fe5aa7269b2 sp 00007ffe63b09530 error 4 in libgtk-3.so.0.1800.8[7fe5aa3d9000+71a000] After restarting the plugin, I can change the brightness again exactly 1 time, after releasing the slider the plugin crashes again.
This has been fixed in git, commit 83f002a0109f3752f033ab90ec73d13ca37b7b29. Please test and re-open if you still have the problem, we are going to release a bugfix 1.5.3 soon.
Or, more likely, we will as planned release 1.6.0 soon including that fix ;)