No, this is a Webmin-side regression introduced when session IDs in the session DB started being stored under HMAC-SHA256 keys.
It is fixed now here and will be added to the upcoming Webmin 2.652 release. If you need to make it work now, just apply the patch below and you’re good:
webmin patch https://github.com/webmin/webmin/commit/c8fb790
@Jamie, please double check this commit though.