It looks like the interface runtime is expecting us to use table.insert and not "key in" array entries directly.
Here is the offending function in my code:
https://github.com/Exterminatus/eso-...e516885616L212
Here is how we should be doing it:
https://github.com/Exterminatus/eso-...72850fa7ff7R12
If you pass a "badly created" array to ESO it seems to cause this issue (in my case, I was sending it to the settings menu via LAM).
Just one to watch out for. There are other plugins besides mine causing this, but sadly my guildies haven't been bothered to nail down which of the plugins are causing their issues.