You are right, it was a plugin conflict. I turned them off 3 plugins at a time, and landed on widget options as the source of the plugin conflict, and verified it a couple of times. This is the free version of widget options. Do you want to report this to the plugin maker, or should I? You know your code better than I do.
Usually what happens when a conflict report is made an investigation is done.
Since the plugin is causing an issue with plugin then its very likely that it will cause issues with not just our plugins but other plugins that deletes in a similar way.
I would recommend getting in touch with their support and let them know about the conflict.