Using J 1.7.2 and k2 2.5.1

Email account activation link returns 500 error.
If user returns to site and logs in front end, gets an Invalid token notice.
USER MODULE link to My Page (component/k2/itemlist/user/54-test) returns 500 error
Auto Tags in EDIT ITEM causes 500 error.
CLOSE "article" in front end end locks.

Ugh. Renders K2 useless as a community building platform.