FrameMaker 2015 crashes when I change conditional text

Hi, all.

I see this is an old problem with questions going back to at least 2009, but none of the answers has worked for me.

We use FM 2015 in a Windows 10 environment.

I have been working in the same files for several years. It's a 750-page document with about 20 chapters. We have PDF and .chm output and I have switched conditional text settings between the two innumerable times over the past two years.

Now, FM crashes when I

1. Select all files in the book and change the conditional tags,

or,

2. Open a single file and change a the conditional tags. I have mif-washed this file and resaved it as a .fm file.

I have rebuilt the book file, also.

This problem arose today for no apparent reason. All suggestions warmly received. Thanks.