Sorry for the delay. It looks like the problem is with the trigger: the group expect it to be numeric. So, even though you have it set to “Active on value change”, it is trying to convert it to a double.
However, the option above- “only evaluate when values have changed”- will accept any type of data. So, I would simply use that, and uncheck the “execute on trigger” option. If you need a more complex trigger, you can add a run-always expression item with an expression that evaluates to 0 or 1, and trigger off of that. The “hasChanged()” expression function is useful in that case.
Ultimately, we should make the “active on change” trigger not sensitive to the data type.