You get that error when something messes up during the procedure for buying silk.
So to buy silk, you have to login, click on the item mall icon, pick how much silk you want, check the "i agree" box, confirm, takes you to paypal page, fill in paypal info, send payment.
IF during any of these steps, there is an error in loading the page (like it dosnt load, or takes you to an incorrect page) then when you go to repeat the process, it will not let you.
I really wish they would allow 2 tries at least. I got blocked 5 days in a row once cause of their crappy site
EDIT:
uBeR wrote:
This happened to my friend. On the forums, MD says you're only allowed one Paypal transaction per day.
Thats not entirely true. I have made multiple purchases of silk per day before. If everything goes smoothly (as in what i stated above dosnt happen to you) then you can buy up to your daily limit for silk with as many transactions as you want.