I found another one that I assume might be failing due to a dieresis in the name: Helicoön ellipticum. It exists in the database, but has no taxon id and clicking the name in search (or hitting enter) brings up the fail whale.
I assume Helicoön ellipticum was brought in via external name providers (it’s not possible to create a taxon with a dieresis directly in iNat). It should be swapped into Helicoon ellipticum, but the taxon change screen won’t find it to swap.
The other thing I noticed is that searching for Helicoon ellipticum in the header search and hitting enter also causes the fail whale. I guess because one of the taxa it’s trying to load in the search results is Helicoön ellipticum.
So I don’t know why all the other taxa in this thread are null, but it looks like a dieresis (and potentially other characters) will cause a null taxon on import from external name providers. And that a single null taxon in header search results will cause a fail whale.