Thanks for your response. I forgot to mention that it's Kentico 11 (v11.0.49). We can’t reproduce the issue as well. The bug doesn't happen on creating a new widget. It happens at random times, usually at the end of the day. When it happens, it occurs to all the online form widgets.
We never had this issue on our Kentico 12 instances. You think it might be an Kentico 11 bug?