Hi Toke,
Also, in the plugin source, there is a mention of term meta table compatibility. This code is constructed based on that compatibility being set to default. I have not been able to find this option though, and I am not sure if it’s in use anywhere, as I only seen a mention in the source. Do you happen to know what that is, and if yes, do you have it set? That might change the outcome.
I have also made a tiny change to the final code, so that the plugin class existence check is omitted. Can you please try this variation as well? Make sure it is in the correct functions.php file, sometimess I mistake the active theme directories 🙂