Hi @quantumodo,
We just remembered something. Does the strange error message look like this image?
If so, you can simply refresh your browser to resolve the issue. This is a bug related to JavaScript, which is used to handle player operations (not for card effects). Refreshing your browser should fix it. However, if the error persists or occurs frequently, please provide us with details on how to reproduce it, including the steps you took when clicking the cards and buttons.
Thank you for your help!