Did you issue "sh crash"? If there is no crash file found, then you need to connect the console and watch what the console prints.
what does "sh run logg" say? Do you have console logging enabled may be debug level. If so pls. disable that. How about http inspection is that enabled? If the connections do get established then, inspection will kick in.
I suggest that you open a TAC case and work with an engineer.
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...