i too have experienced this problem…and i note that quite often today i have experienced issues with the connectivity to webmin.
I am running it on a small instance (600MB RAM and 1VCPU@ 2.5 GHZ) although my install is not needing to provide DNS or MAIL server. Essentially this will be just for webhosting sites such as Wordpress.
I have managed to get back into my server by accessing it from ssh and manually restarting webmin.
I am after some hints on minimal webwin/virtualmin install capable of hosting wordpress websites (I am using virtualmin pro)