I encountered this error late this evening while grinding in The Lost Grove zone.
I use OperaGX as my browser. I've tried leaving the zone and re-entering, closing and opening the tab, and even resetting Opera. Nothing seems to alleviate the issue. I have never had this issue before now.
This error pops up after I enter my initial moves and click 'end turn'. After I clear the error, I can continue the battle as normal. It is just a little irritating to have to close it at the beginning of every battle.

Thank you for your time! I am enjoying the game so far!

Hello.

It sounds similar to this issue:

https://www.sylestia.com/forums/?thread=105393&page=1#4

Could this be the issue?

Yes, that was the issue, thank you. I'm going to keep any addons disabled from now on.
I saw that thread but it didn't quite look the same, which is why I brought it up. Apologies for the trouble, and cheers.