Well the stuff I pasted in to my post doesn't seem to have made it. Pretty sure I tracked down the source of my trouble, it's in the following code block:
\[
\begin{align}
y=
&c_0\left(\hbox{\{ ans_rule(2) \}}+\hbox{\{ ans_rule(5) \}}x^2+\hbox{\{ ans_rule(5) \}}x^3+\hbox{\{ ans_rule(5) \}}x^4+\cdots\right)\\
&+\\
&c_1\left(\hbox{\{ ans_rule(5) \}}x+\hbox{\{ ans_rule(5) \}}x^2+\hbox{\{ ans_rule(5) \}}x^3+\hbox{\{ ans_rule(5) \}}x^4+\cdots\right)
\end{align}
\]
If I remove the align environment then everything seems to work.
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errors in answer blanks in webwork 2.7
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