Looks like a database issue to me, because some articles are missing title and description, some articles still have their title.
So, i was able to buy / checkout this article which still has its title but is missing its description: https://marketplace.secondlife.com/p/DT-Dead-Tattoo-Maitreya/9227314
What i actually wanted to buy in the first place is this article which is missing both title and description: https://marketplace.secondlife.com/p/Expressive-PonyHUD-make-Pony-or-Horse-Sounds-and-emotes/2551235
So, since the cashier needs to display the article's title on the "bill", the server gets an error if the title is not available.
Btw.: I never got an access denied error yet, i also tried clearing cookies and stuff, did not help.