Hi!
I just posted the same issue yesterday 
, so probably is something recent.
I found the problem is not something related to Mono, but seems caused from data reading from the database.
I found a workaround: before starting duplicati, I cleanup the “server-ssl-certificate” option inside the Duplicati-server.sqlite db:
update option set value='' where Name='server-ssl-certificate';
You should do every time, before starting the server.