Form / Textarea / MediumText数据类型[错误]

I want to ask something about my webpage.

I Got problem with posting some post.

Case : When i start to post with fewer words, it worked and added to database. But when i start to post with longer words, like paragraphs or something it becomes error and the post didn't go throught to the database.

Here the first successful post case :

Its like 10 to 15 words inside or 3 to 4 paragraphs and it would work.

But when i type a post with 10 paragraphs or more it would become error :

The error looks like this :

it said i don't have permission to access the file. But, why? Before i post the long one, i got permission to post, how can i get no permission when i post with the long one?

Specification : -Data Type to store the text : MediumText -The textbox element im using is :

form code :

and the action page will get the text with $_POST['textarea_name'], see the picture below :

I wonder if there is a limited character for element or there is a limited character for MediumText data type or something from the element can't handle too much text? Thanks for whoever answer this. I need your help so much guys :)