Frans,
If I understand it correctly there are two different "plugin Managers", and I assume you have tried to installed the second one named "Domoticz Plugin Manager", correct?
Domoticz PluginsTitle

Can you describe how you tried to install it in a bit more detail and please also the "trace" leading up to the "startup variable"?

In ClearOS the Domoticz the executing (working) directory is (and should be) /usr/share/domoticz . However, the userdata, webserver files and database pahts are different ( /var/domoticz ) and this is set at the startup of Domoticz. In Debian based systems (such as Raspberry Pi) it looks different. My assumption is that there is some "hardcoded" path in the plugin modules part that is assuming a Debian file structure which is then causing the trouble.

I am pretty sure the plugins should be installed in /var/domoticz/plugins/ .. Also, make sure that domoticz is owner of the files (recursive in all sub-directories)

I might be mistaken, but I think the bug-fix that was made to Domoticz Plugin Manager inCommit #16 was not the best... "StartupFolder" should have been "UserDataFolder" instead (two places). Also, they seem to have forgotten about fixing a row in plugins-manager/manager.py in row 80 also. it says:
which I guess mean it takes current working directory (set to /usr/share/domoticz ) and add /plugins so the final path to where it "thinks" it sits is "/usr/share/domoticz/plugins/ ". Which obviously is not correct.

With Domoticz Plugin Manager installed in /var/domoticz/plugins , owner set to domoticz, and with line 63 and 79 changed so it says "UserDataFolder" instead of "StartupFolder" (and hardcoded the row 80 in manager.py to "/usr/share/domoticz/plugins" it seems to work a bit better. I do not have time right now to "install" a plugin from the menu. But you can try this first if it is possible to experiment on your side.

/Fred