hey - i'm new here (first install / first install problems -.- you know)
what i can see so far is: fail2ban will not work for http requests as long as the login page returns http code 200 for an access denied case.
fail2ban works with error logs and wrong return codes prevent any use of it.
its not the fault of the environment (nginx etc.) as logs only contain request urls and not the post data.
if the omv implementation is modified, fail2ban and any other log crawler will work again
the ssh part is a different story - and i think it's not really part of the omv service.
you would have the same problems on any other installation