OC Final test, it seems tags is the issue for the 500 Internal Server Error.
If the Tags are not in Latin/English characters, we're getting a 500 Internal Server Error.
For example, if we add a Korean or Japanese tag, the new Thread will return a 500 error.
Use this as an example: 유니스 and ユニス.
Update (7 hours later): It is also happening when creating a new post under the “Microblog” section.
Another update: using non-Latin/English hashtags in the body of the post under “Threads” section works fine. Just not in the “Tags” field.