En Masse Entertainment is currently working on a resolution for the issues. Updates will be posted to the official TERA website, Facebook and Twitter pages. Stay tuned and well update you when we have more.
Update: En Masse Entertainment has corrected the account issue of plagued accounts.
We've pinpointed the cause of the inaccurate remaining subscription time, an issue that's affecting about 2000 accounts. We've configured these accounts so that those players will be able to continue adventuring in TERA while we work to resolve the issue on the backend, where it won't affect you or your friends and guildmates.
Thanks for your patience!
Source: TERA Update on Account Issues