Thanks for the info re underscore. An old header file was the problem.
As for "dots" in homework set names I'll give you an example.
(I am not trying to solve an issue here - we have stopped using dots and that is just fine.)
Homework set Section_1.1 containing the following problem:
generates the issue.
Making a hardcopy gives the following error:
! LaTeX Error: Unknown graphics extension: .1prob4image1.png.
Its pretty clear that latex is confused over the two dots in the filename used for the image file,
Note this only arose for dynamically generated images.
I thank you for helping make webwork possible. It is great.