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I have just tried xdonations with 2.2.3a-Final version installed on local server and IPN works properly!
So, I think that in my 2.0.15 version installed on remote server, IPN doesn't work because of changes made by me in some files for installing new modules or features.
But I don't understand which file or which module could be the cause of malfunctioning.
EDIT: Also, I have tried on new local installed 2.0.15 version, with only system and xdonations modules installed. Unfortunately, IPN doesn't work.
So, I think there are some problems with 2.0.15 version.