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,
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.
You've already tried two of the things we would have recommended. What's left is rebuilding the preferences (see https://www.rockymountaintraining.com/adobe-framemaker-resetting-preferences/). And it may be worth reviewing your Windows build. Did this start happening after a recent Windows update?
Also, note that Windows 10 is not supported for FrameMaker 2015 by Adobe. Support for Windows 10 was introduced in FrameMaker 2017 and quite a few changes were made in FrameMaker for Windows 10 support. Plus there have been many bug fixes and many improvements since FM 2017 on many levels, especially in FrameMaker 2019.
My suggestion here is to download the 30-day free trial of the 64-bit FrameMaker 2019 version and test it with your documents on Windows 10.