MyBB Community Forums

When i try to do a new post or a new thread...i have a problem in text box. When i type Enter to go to the next line the problem appear. How can i solve this? where e which files i can find this?

which editor you are using ? provide forum url & a test user account so that someone can check it

Can you try using a different editor in Theme settings to see if other editors are working properly ?
Most probably it is an issue with your theme or the custom theme editor.