Since you said periodically does that means same ssl clients can successfully connect some days and cannont connect other days? I would try looking at ASDM logs specifically stick with a webvpn user that cannot connect and see the logs..
Also ensure you are not going over your SSL vpn licensing limits just to rule this out, but it sounds more of a client browser issue, in any case start narrowing down by using above link.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...