To allow you to do things like quotes and so on, you need to go to your profile, and allow BBCode. There is a box available to tick.
If not, then as you post, there is also a box you can click. Always allow BBCode, and your posts will work as they should.
I'm not quite sure why this has occurred, it might be something to do with changing the actual forum, but if everyobe allows BBCode, everything will work fine.