Geraldlzc does it happen with any post content? If it's only with a specific text, can you copy-paste it for us? Does the same text work fine on Flarum beta 16?
Based on the message in the screenshot, I assume you are creating a discussion, not a post. Maybe you also have some special characters in the title?
Are you able to reply to discussions?
If those tips don't help, can you share your php flarum info output so we can see which extensions you have enabled?
