I have a simple contact form on a website that has 2 text fields, 1 textarea, and 1 hidden field.
For some reason, all the fields POST to a PHP script except the tex
You only need to add an ID value to the form and then add the form attribute to the textarea with the form ID value
<form id="sample".....>
<textarea name="aba" form="sample".....></textarea>
</form>
I once experienced an error similar to yours. What helped me was to use different id and name parameters. Try and see for yourself, because you have them identical here.
Though in your case you don't have the textarea set to disabled, the reason I found this post was because I wasn't getting a value from a textarea that was. So here's a note for anyone else with that issue.
To POST the value from a textarea where you want the field to be non-editiable, use readonly instead of disabled - either directly in html or via setAttribute in JavaScript - and then use CSS to highlight it, eg:
textarea[readonly] {background-color:#F0F0F0;})