It is not possible to install a custom extension in Extensions > My Extensions > Upload Extension:
PLESK_ERROR: Error: Unable to install the extension:
Execution of /usr/local/psa/tmp/modulefXP2il/plib/scripts/pre-install.php failed with exit code 199 and the output:
Site error: the file <b>/usr/local/psa/tmp/modulefXP2il/plib/scripts/pre-install.php</b> requires the ionCube PHP Loader ioncube_loader_lin_7.1.so to be installed by the website operator. If you are the website operator please use the <a href="http://www.ioncube.com/lw/">ionCube Loader Wizard</a> to assist with installation.
Plesk 17.8 is used on the server:
# plesk -v | head -n1
Product version: Plesk Onyx 17.8.11 Update #11
Starting from Plesk Onyx 17.8, extensions protected by ionCube are not supported.
Contact the developer of the extension and ask them to encrypt it using our source code protection: Developing Extensions for Plesk: Encoding the Source Code.
If you are the extension developer, please submit a request to firstname.lastname@example.org to encrypt the source code.