I stumbled upon this thread when encountering the same problem. For me it was the Finding True Love After a Breakup scenario that made the error. I had put the scenario household in my "unplayed household" category when the error started. I found the scenario household again and deleted it completely. That was when the error stopped happening and I could save as usual. Hope this can help someone!
Thank you so much for this thread - I have the same issue and it was resolved after realising a household with an active scenario was moved to my “Other Households” - I cancelled their scenario, and now everything works again!! Such a relief
