I uploaded both, the Cronjobs and the InstantSSH plugins.
Then, I installed the cronjobs plugin, but it shows the following error:
An unexpected error occurred:
InstantSSH::JailBuilder::MakeJail::createJail: Traceback (most recent call last): File "/var/www/imscp/gui/plugins/InstantSSH/bin/makejail", line 118, in import magicImportError: No module named magic
After installing the InstantSSH plugin, the Cronjobs plugin also installs without any issues.
Maybe this should be mentiones as a dependency on the wiki page.