Thanks. We needed Xdb.Tracking.Enabled turned on as well to be able to use/track some personalization stuff but we don't need all of xDB for that.

Yes, but the field "_group.guid" doesn't exist :)

Well, in short it's because the expression is not actually executed as such. Sitecore attempts to translate your C# expression into a Lucene query. From your log you can see that it translated x.ItemId.Guid into _group.guid - a field that doesn't exist in your index.

The log you posted looks correct, at least if you are using the default index configuration. Are you using the default configuration? If not, could you show your index configuration? I'm mostly interested in which fields are included and their names.

There seems to be a data mapper for both NameValueCollection and Dictionary<,> types (and has been for a few years) which both uses HttpUtility.ParseQueryString() under the hood, so theoretically your code should work unless you are using a very old version. Have you tried using Dictionary<string, string> instead?