This error message is generated when trying to add a variant product to a purchase order cart after features have been selected:
Error calling event: org.ofbiz.webapp.event.EventHandlerException: Problems processing event: java.lang.IllegalStateException: This object has been flagged as immutable (unchangeable), probably because it came from an Entity Engine cache.
1. Add new feature "Gold" as COLOR Feature Type to Widget Features Category
2. Change WG-9943 virtual variant method to "Feature Tree Generation" on the product page
3. Add new "Gold" feature to WG-9943 on feature page of product app
4. Create new purchase order from BigSupplier for WG-9943
5. Select the new feature "Gold" as the color
The error is generated after trying to add the product after all features have been selected.