Hi @ll,
i debbuged that problem last week and found the cause of the issue. The bug is reported and will be fixed in 1.6p19 with the following werk:
The problem is an import in cisco_webex_teams_notifications mkp that is shipped as a checkmk extension:
/opt/omd/sites//local/share/check_mk/web/plugins/wato/webex.py
Line 11
import cmk.gui.mkeventd as mkeventd
If you remove that line or comment it out, the deletion of timeperiods should work again.
hope that helps 
Andre