Cannot Add New Condition Set Code to a Rule

 Hello

I have created a Tax Status rule and added one condition set code successfully.

I go back later to add a second code and the Result of the first code now shows an error message 'Select a Valid Value'.

If I reselect the Result for the first Condition Set Code I can then add a second one. However, next time I go in, both Condition Set Codes show the error code for the Results.

The Tax Statuses selected are definitely enabled and valid for the date of the rules.

It only seems to happen on the Tax Status rules. all other rules are OK.