Thanks for reaching out to our Australian community and sorry to hear you're having trouble posting!
I have seen this before - in the past it's been due to the formatting of text I've copied and pasted when posting to Seller Community. It could also be due to characters/symbols in your post that the form is interpreting as HTML code.
Recommendation #1: Copy and paste the messaging without text formatting
Recommendation #2: If applicable, and if the first recommendation doesn't work, can you try posting the message without special characters outside of what you's normally use in text conversation (eg question and exclamation marks should be fine)?
Hope this helps, let me know how you go! Also tagging @tranguyen for visibility!