Thank you for coming back to me. I had great hopes after I saw in the changelog that "a number of issues with Relevanssi" were fixed in Views 2.9.
But unfortunately, this issue does not seem to have been fixed. I updated everything, cleared all cache, rebuilt the Relevanssi index - and still the same. Tested on 2 sites, a simplified case produces the same error.
Apparently, there are many compatibility issues with Relevanssi (some of which have been resolved, yes, but still Relavanssi and Toolset don't seem to play nicely together). So, sometimes I wished Toolset had some native relevancy tool not relying on third-party products.
Indeed, Nigel. As far as I can tell, it works correctly now. In my quick test, both custom fields and taxonomies behaved correctly. Fantastic! Thank you!