You don’t need to reinstall old versions. And, you shouldn’t make any config changes to try to work around it.
There have been several posts about it (though I’ve been unable to reproduce it locally so I don’t actually know what is happening). I suspect just restarting the webmin service will resolve the problem. Other threads about the problem have other suggestions, e.g. After latest Webmin upgrade configuration wizard - #7 by shillongserver
We’ll roll another update once we figure out what’s actually causing it. Seems to be specific to the deb package version (I think, some folks have neglected to mention their OS/version), which is what makes me think it’s not restarting for some reason.