You can report the issue in gVecotrs Forum. Make sure you've attached the exact content of the post which has the problem: https://gvectors.com/forum/

Disabling the plugin makes everything work again.

From what I can see the DIV is simply empty and not produced by the forum.
However when I edit a post, everything is there and shows correctly.

Here another instance where this happens: link.

This is what the post looks like to viewers:

having dealt with this before, even with other forum/editors/comment-posts, I am aware that some character combination can cause issues. In the past I was always able to find what triggers it.

On that note:

1) The "<>" button has been used for this code.
2) Tired posting it here, also using <>, and that works fine.
3) When editing the post, the page every now and then freezes up (Chrome and Opera anyway) - nothing works anymore (button).

Buying yet another addon (I already have a few) is not an option for me at the moment unfortunately 😞 

I have not yet found a solution unfortunately.

Posting a code directly in the editor can always affect the topic layout. It'll not case security issue but it may crash the HTML of your forum page. That's way we have added <> code button. In many cases the crashes happen even wuth <> code button. So you can check out the wpForo Syntax Highliter addon: https://gvectors.com/product/wpforo-syntax-highlighter/