Okays. That might explain it. Grouping is for now only possible by Category or Post type but not by taxonomy. The search is trying to retrieve a category name for a taxonomy which probably results in a fatal error.
The category and taxonomy search is a fairly new feature so I'm working on some additional things regarding them. I'm going to add your request to the TODO list right now. I'm almost done with the next major update (which contains mostly compatibility updates), but I guess I can work out something, since grouping needs an update anyways. I can't promise it's going to be exactly how you imagined, but I will add as much flexibility as possible 😉
Will contact you in a few days with an update on this.Best,
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