Hi @perdrix,
Welcome to the forum!
Probably the issue here is that none of your installed desktop environments have a default configuration for them in ThinLinc. If you check the file /opt/thinlinc/etc/conf.d/profiles.hconf, you’ll see which ones are available by default.
You can either install one of these DEs, or you can add a new stanza to profiles.hconf for whatever desktop environment you would like to use. My colleague @matre has written an article which might help with this here:
If you need any assistance though, just sing out.