In rules builder, I select “On a schedule(cron)” and then click “Build”. Try as I might it will not save the expression back. Clicking “Done” ignores the content created and just leaves message “Error: expression is empty” message. Might this be a bug?
I just did it and I’m not seeing any error. File an issue at the openhab-webui repo and follow the instructions in the template to gather all the information necessary to diagnose the problem.