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I’ve now managed to resolve this, but thought I’d log it in case anyone else encounters this bug.
The species Beyeria sulcata already existed in iNat, along with one variety (out of four). I added the other three varieties and created taxon framework relationships for them. However, for one of those 3 taxa, I accidentally set the rank as subspecies instead of variety. When I clicked edit relationship to fix this, it gave me the whale shark error. Attempting to edit any of the three TFRs I’d just created gave me this error. When I tried to edit TFRs created by other users, or those created by me earlier today, they all worked; it was just these three Beyeria ones.
It turns out that when B. sulcata was created/imported into iNat, it was grafted to the beetle genus Beyeria instead of the plant genus (and thus the three varieties were also under the beetle genus). When I regrafted B. sulcata to the plant genus, I was then able to edit the TFRs as normal.