PPA 11.1 assimilation of Plesk 11.5 fails, despite Plesk node having all necessary packages and services.
Incompatibility of product versions. It is impossible to assimilate higher version (11.5) to lower (11.1) as there are some features in newer version that can not be operated by the older one. As a consequence assimilation fails.
Perform transfer from the supported platform.
Information regarding the transfer scenario could be found in the following page of documentation.