[A17120] Assertion failed: aptFalse, file ../src/forsirtns.cpp, line 6025

Hello,

I've just been handed a group of technical manuals, in-progress, and been asked to finish them. When I open one of them, I am directed to browse for and link a DTD/Stylesheet. I find the correct one (confirmed it a dozen times, it's the right one!), link, and then immediately get a pop-up error:

"[A17120] Assertion failed: aptFalse, file ../src/forsirtns.cpp, line 6025"

Any ideas? Thanks!