Just had another one of these.
At the time, I was moving down the ‘Index’ in the bottom left frame. You will see that the index is filtered on a group. This group is constructed using a saved criterion, namely that number of spouses >= 2. I had been doing this for some time with no problem before the problem did in fact arise.
I should add that RM did not freeze when the error message appeared, either this time or in the previous instance that I posted about. However, after first getting the problem, it then came up repeatedly when doing the same thing, sometimes on the same record, sometimes not. After closing and then restarting RM9, I continued with the same process without any problems.
Alan