Recently
it turned out that under the new rules of PayPal in Russia, donations (using the “Donate” button) cannot be accepted.
Here is a Paypal support message:
Open a PayPal account at this time.
in Russia for receiving donations is impossible due to different financing rules
There was a suspicion that donations in any form were prohibited, for example, transfers from Flattr, which was confirmed by the support of PayPal:
You cannot accept transfers from FlattrNetworksLtd, as this process is still a donation offer.
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Flattr himself did not comment / investigate the situation:
If they say that, it's how it is. I'm afraid. It's probably due to some financial laws changing in Russia. Usually it can not be done.
Conclusion: withdrawing money from Flattr, without breaking the rules, is possible only through Skrill (and it's easier, probably, not to withdraw at all), and when receiving a transfer to PayPal from other sources, you need to make sure that this is not a “donation”.