So, I'm not sure if this was mentioned anywhere, but i'm gonna post it here for anyone with the same problem, as i couldn't find any solution.
I kept struggling with starting preclear (running 2017.11.01 preclear plugin on 6.3.5 DVB edition) from the webui, it always got stuck on 'Starting...' and nothing. The status/log window that is supposed to pop up always just closed immediately. I could however run the script from terminal with no problems. In the end it turned out tmux was at fault. I think the version in nerdtools is the same as the one shipped with the plugin, however the one from the plugin doesn't start (tmux: /lib64/libc.so.6: version `GLIBC_2.25' not found (required by tmux)). Indeed my glibc is v2.24, so that makes sense. The tmux from nerdtools runs fine, so my solution:
1. uninstall the preclear plugin
2. uninstall tmux
3. install tmux again from nerdtools
4. backup /usr/bin/tmux
5. reinstall preclear plugin
6. restore /usr/bin/tmux from backup
This seems to fix the problem with tmux not starting and preclears are running once again.