1. It seems like a different problem.  Paypal is telling you that you are sending Null values  and me that I am sending unrecognized variables.

2. They only say that this is a known issue. Sounds to me like and very old issue not something that occured during this weekend. Or am I wrong?

I do not really understand the fix.

1. Does it help for you to delete products?

2. what standard buttons did you ad?

Well ... there are a lot of people caring here ...problem is that  it is a huge weekend. Most of us probably does not spend it in front of a forum :-)

 I tried to make a new portal on one of my servers (dnn 5.2.1)

added the store module and added a single product.

Still the same problem after checkout with paypal crashing.

So it does not seem like the amount of products affect the outcome for me.

Santos --

I found your problem and have a solution:

Paypal is displaying the "Internal Server Error" because you are sending to many unsupported variables or may have the following attribute in your <form /> tag: enctype="multipart/form-data".

I would use javascript (in an OnClick event)  to remove the enctype attribute from the form before posting to paypal.  Here's the submit button that I use: 

<input border="0" type="image" alt="PayPal - The safer, easier way to pay online!" name="submit" src="/paypalButton.png" onClick="jQuery('#Form').removeAttr('enctype'); this.form.action='https://www.paypal.com/cgi-bin/webscr';this.form.submit();" />

I use jQuery here to remove the attribute but you may also be able to use "this.form.encoding = ' ' " but I haven't tried it yet.

I'm no longer receiving the error form paypal and everything works like a charm.

Hope this helps!

Dave Romero
Tsuvo.com