When deleting a mailbox user in the latest version I get the following:
Failed to save mailbox : Missing file to open at htaccess_htpasswd::../web-lib-funcs.pl line 1565
Running Webmin 1.217 and Virtualmin 2.67 with database backup modification. It only happens on users with a Subversion account, I think (maybe DAV?). You can see it happen on virtualmin.com if you have trouble replicating this issue. Just create a test user in the virtualmin.com domain with subversion access, and then delete him.