Hey Nuxwin , me again 
I tried to install the DebugBar without success on release 1.5.3-2018120800
On the plugin page, it's marked "Version compatible with i-MSCP Serie ≥ 1.5.x (version ≥ 1.5.2)"
but, in the info.php of the plugin :
'require_api' => '1.5.1'
But, if I'm not wrong, this plugin API will be at that version for the upcoming release ?
This is my imscp.conf, maybe it's wrong, or not :
QuoteDisplay More## i-MSCP version info
Version = 1.5.3
Build = 2018120800
CodeName = Ennio Morricone
PluginApi = 1.5.0