/usr/lib/mono/4.5/Mono.WebServer2.dll is in mono-xsp4-base 4.2-2.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/Mono.WebServer2/0.4.0.0__0738eb9f132ed756/Mono.WebServer2.dll