I received good answer from PayPal Support about this problem. I am verified. I quote the answer. It gives light for many cases:
Thank you for contacting PayPal regarding a payment that you want to send to your friend.
I have reviewed your PayPal account and can see that your payment of 6.00€ has been stopped by our internal security system.
To provide one of the safest online payment methods PayPal reviews every payment before it's approved. Occasionally, we may block the use of a particular card or your balance if there may be a higher-than-normal level of risk associated with the transaction.
When a payment is blocked, this is not a reflection of you or your account activities. Our internal security system is an automated system which reviews all of the details of every payment that is made through PayPal. It has been designed to recognise patterns that could result in buyer complaints, unauthorised claims or chargebacks. This is similar to the way your bank or credit card issuer approves transactions.
For fraud prevention reasons, I cannot see what aspect of this payment has caused our internal security system to intervene. Neither can I override or manually change the security settings to allow your payment to go through.
I can, however, give you advice on the steps you can take to reduce the chances that our internal security system will block any future payments.
Here are some things that you can do that may help:
We regret any inconvenience this may have caused and we advise that you contact your seller and inform them why the payment has not been completed yet. If the seller accepts payments through PayPal they will be familiar with your situation.
It is my pleasure to assist you. Thank you for choosing PayPal.