[MucMan]

Filter and folder type approaches to custom nodes can not be mixed. The filter type has more variety and complexity of rules (like smart playlists) but these are only applied at the level of the filter e.g. filter artists by path (finds artists with songs of that path) then click on an artist you see all the albums then songs of that artist regardless of the path. Folder type have limited criteria e.g. a genre, a source etc. but that criteria applies as you drill down artist, album song.
 

I would like to create a library node that contains a source information to an external path (so that I can select e.g. all my classic music) and that shows albumartists only.

Assuming you are using v18, I think folder type custom nodes with source criteria will achieve what you want. This is based on having multiple music sources set up. You don't have to move your music files into a new layout, just add music sources that point to meaningful subsets of it, a source can even have multiple paths. Say one of those sources points to your classical music called "ClassicalMusic" then create a folder type node

Code:

<?xml version='1.0' encoding='UTF-8'?>
<node order="1" type="folder">
    <label>Classical Album Artists</label>
      <icon>DefaultMusicGenres.png</icon>
    <path>musicdb://artists/?source=ClassicalMusic&&albumartistsonly=true</path>
</node>

If you need more help with this then just ask. Folder type nodes are an (as yet) undocumented but powerful approach to achieving some views of the music library.

You don't have to move your music files into a new layout but you may find this thread interesting https://forum.kodi.tv/showthread.php?tid=345364 

Thank you David for your solution, this is working perfectly. And thank your for posting a dedicated complete example, this is extremely helpful for non-experts like me.
I wasn't aware that I can use that easily the source field in the <path> query of the folder type node (all the examples I found before only used it in the filter type nodes with "long" criteria definition like <rule field="path" operator="contains">  <value>Classical</value>   </rule>).
On a first glance it seemed to me that only the filter type nodes with the possibility to create complex rules are flexible, but now I get the feeling that also the folder type syntax is very powerful if you know how to use it.