Hi there,
I have 2 freshly installed Virtualmin servers running Ubuntu LTS 18.04 having the same Problem when deleting website aliases.
I get this error message and afterwards I cannot recreate that specific alias without going into the /etc/sites-available and manualy deleting the alias from there.
The error is always the same (line 3216).
Here the copy/paste:
Deleting mail aliases ..
.. done
Removing from target virtual website ..
.. Apache website failed! : Unescaped left brace in regex is illegal here in regex; marked by <-- HERE in m/^\%{ <-- HERE HTTP_HOST}\s+=(webmail|admin)\.bomdeluxe.lu/ at /usr/share/webmin/virtual-server/feature-web.pl line 3216.
Deleting server details for bomdeluxe.lu ..
.. done
I think this is a BUG because on my older servers (Ubuntu 16.04) I do not have this issue.