Plesk update fails with the following error:
ERROR: The required version '3.15.15' was not found on the server.
This could happen because of configuration error at the download site.
The MD5 checksum of the /var/cache/parallels_installer/parallels_installer_CentOS_5_x86_64 file does not match the value specified in the configuration file.
The version and file name in the error message may vary.
This is caused by new Plesk 'autoinstaller' capability for self-upgrade prior to upgrade of the product. If 'autoinstaller' was not mirrored, it leads to this error.
Please follow the instruction described in the article [KB112166]How to upgrade Plesk with the latest version of autoinstaller