Quite by accident, we noticed that some stylesheets can have issues when upgrading to 7.0 from 6.1. A day or so ago, Liz was working with some stylesheets that hadn't been touched since mid 2016 when she published her books. When she opened the stylesheet in 7.0 M080, Styler crashed fatally. She could not open the stylesheet at all.
After some investigation, it appears that the crash is related in part due to the introduction of support for MathML and the introduction of a new stylesheet element between the release of Arbortext 6.1 M020 (May 2014) and Arbortext 7.0 M080 (Sept 2017). She knows better. We always tell customers never to jump a stylesheet that far without vetting, but she has multiple versions installed and admits that she "just wasn't thinking about it."
This problem affects both DITA and Docbook stylesheets. We have found it in several customer instances as well as in our own stylesheets.
Liz posted the method for fixing the problem on the Arbortext Mastermind members forums. There are specific steps and pictures because the fix takes some non-traditional methods. That's where the write-up will reside for now. Not that she's running out of topics, but it looks like Liz has a new troubleshooting topic for her next Styler book. We love adding to her stack!
In the meantime, If you're migrating, upgrading, or otherwise concerned, you should validate your stylesheets and test all the stylesheets you're using. If you come across the crashing problem, you can fix it yourself if you're careful. We can do it for you as well. Just let us know if you want us to do the testing for you or fix a broken one you find.