OctoPrint no longer includes an embedded slicer as a bundled plugin. And that plugin was a really old version of Cura anyway. Although this person on the OpenSCAD forum suggests that they'll be creating a workflow of pushing the STL directly over to OctoPrint this doesn't seem like a great plan because of the age of the plugin and version of Cura.
The BAD REQUEST is very likely because you're using the wrong HTTP verb.
I can't really speak for foosel but as I remember the story, OctoPrint began as some sort of CuraEngine sort of thing and then grew from there. This would be as much as a decade ago. Rather than putting any more time into the slicing aspect of this she's put her effort into other aspects of the infrastructure.
As early as three years ago I think most of us weren't even using the built-in CuraEngine, preferring all the cool new features of the stand-alone version of Cura. So you're talking a long time ago back in the v15.x days.
For me, personally, I'll likely want to continue to bring it into Cura. I'm really into my Cura settings and what I've done to get good adhesion. Abandoning that and trying to then work with another slicer would mean that I'd have to play with all that again.
So I'm not really the best person to ask. I think I might have used OpenSCAD before but it was a long time ago. I like the way it works. I think I just fell into a Fusion 360 workflow and have built up lots of models in that.
Yes, it is very much desired, I simply lack the time.
There was a significant amount of time invested into trying to adapt the Cura plugin to newer versions of Cura Engine and it was a nightmare since it was pretty much shooting on a moving target back then when. Things have since settled and today it would probably be easier, but I no longer have the help I had back then and have to rely on external contributions here. If you want to create a slicer plugin for modern Cura Engine that would certainly be appreciated and I'd also be happy to give you a repository under the Github OctoPrint organisation for that.
The challenge will be the handling of slicing profiles since in the newer Cura Engine they are hierarchical and depend on other profiles. My suggestion would actually be to move away from trying to migrate the profiles to an OctoPrint-internal format like the old legacy plugin does but instead use the native profiles. Management will be a bit of a pain though.
I had a look on the old plugin yesterday so my idea is to import the profile files as they are.
Use them as they are and add a front end with one line per option to change or add settings to it.
I think that will be the 'easiest way' to go.
The hierarchy can be a problem, I have to check this.
