Since few days some useful KDE apps hung when they are started. Okular is in this case, which is quite painful as I use this app many times a day. I used to reboot my workstation in order to resolve the issue until now. But, I got fed up with reboots. So, I started dig in order to resolve the issu.
The first move was to strace okular, by strace okular. After few seconds, the last line on the terminal was connect(10, {sa_family=AF_FILE, path=@"/tmp/fam-glennie"}, 110). This line is quite clear : fam stands for File Alteration Monitor. I switched to gamin few years ago beacause it uses inotify/dnotify under linux.
My first try was to kill gamin_server, and okular started instantaneously. So my issue was linked to gamin process. And now, I've a solution to resolve my issue without rebooting.
FYI, a bug is filled in debian bug tracker.