From b2evolution manual

This manual page is outdated. NEW PAGE: Html Validator in b2evolution v5+.

The HTML validator checks the HTML of your post to see that it is valid.

It also rejects numerous tags, such as <object>, (which means can't post YouTube videos, although there is a plugin for this).

To disable the checker, edit the file blogs/config/_formatting.php, and change

$use_html_checker = 1;

to

$use_html_checker = 0;