Mono Releases says * Mono 5.4.0 (05 Oct 2017) which does not yet have a fix for mono crash seen here:
Removing source folder results in inaccessible UI and endless backup-loop
Resolutions are not always stated, but in the below someone fixed a problem like that by upgrading mono:
Duplicati crash after running backup
EDIT:
I’m not exactly sure where your mono stack trace shows up, but at least one report found it in their syslog.