UnivariateTaylorSeries(Expression Integer,x,0)

you should read that it is actually R[[x]] where R is (basically)
allowing anything as an element. What you see above "x x" is the
following: The second x is from the powerseries. The first x is from the
coefficient ring. They are not the same.

The problem is that Axiom allows to construct such confusing things.
You should NEVER put yourself "Expression Integer" into the argument of
UnivariateTaylorSeries or UnivariatePolynomial. And if you see it, you
should be VERY careful.
And that is the way to go. You have to build
UTS(R, x, 0) with an R that fits your purpose but does not allow the
symbol x.

A quick workaround for your problem is probably to choose a name of the
variable of your Taylor series that does not appear anywhere else. Say,
for example,

y := taylor(myNowhereElseOccurringSymbol);

Ralf