Hello @-niels-,
To be fair the new Arnold renderer came out for 2017 a bit before 2018 launched so I thought it was worth asking as both have template managers. No worries. Couple things.
1) Could we just uncheck "Open on Error"? Problem solved? You can always get it back on the Rendering menu. The dev team is aware of the errors being annoying and it's on their radar.
2) It's not a defect but it is annoying. It wasn't meant to read the material editor slots but if it's something you want you could do a feature request. There is some history of the Custom UI for Design loading presets for ART, so there is a precedent there. Generally the idea behind the material editor is that it's a palette and the libraries are used for summoning materials as noted by Vusta.
3) There is an issue in here with something else which I found when I was looking for a workaround for you. Specifically, the templates aren't loading workspace startup scripts properly which is annoying because it would have solved this issue right out. I have logged this.
4) So that leaves us two main options as workarounds. I'm afraid that neither of them solves the error message coming up, but they do solve the problem of getting materials into the mat editor that are the Arnold presets.
The first option is that you could just use the attached script and run it when you want to fill you material editor with those materials. Drag it into the Max viewport and drop or bind it to a button and you're all set. It's by no means an elegant script as I'm no programmer but it does work. Don't forget to update the path in the script before you do it.
Or, you could make a new workspace that fires off a script on activation. To do this, create a new workspace and tell it to run the attached script on open. (Update your path in the script.) It does mean that you have to switch workspaces to something else and switch back to autoload the materials into the mat editor slots because the script loads the mats when the workspace is activated. At home I have this set for every major renderer as a different workspace with my favorite mats loading in when I switch between them. (Created to test this script, kinda neat though.)
It's a workaround, but does it help with your issue? Keep in mind this is not an official Autodesk script and other support specialists can't really run support on it, but I tried to make something that solves your issue. Could this work for you?
Best Regards,