Pay Pal IPN error & bad request

[juagicre]

VirtueMart 2.0.1 & Joomla 1.5.23

I've been doing a lot of testing with my paypal sandbox without any problem.
Once all the things worked as I wish, I just tryed to buy like a normal client would do and then wired things started to happen, first I got several mails (10 to 20 mails) like this from my own store to my own store (!?) whenever I try to buy something:

subject: error with paypal IPN NOTIFICATION
message: VMPAYMENT_PAYPAL_ERROR_IPN_VALIDATIONINVALID

Even I was able to buy the first try but then, after that, whenever I "confirm order" I get the following screen:

400 Bad Request
Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.
Cookie: KHcl0EuY7AKSMgfvHl7...

I tryied cleaning the cookies, the cach, and all the navigation history and so on but no results...
I've been looking around this and I could not find anything... what is happening here? how can i solve this? I did not touch anything fom the payment plugin...

Thanks for your help in advance!