Potential fix reported downstream on Sage ticket #13973:
~~~~~~~
 a/share/linearalgebra/matrixexp.lisp 20131007
04:37:12.000000000 +0100
+++ b/share/linearalgebra/matrixexp.lisp 20140516
02:16:09.112011893 +0100
@@ 138,8 +138,8 @@
(print (ratvars = ,$ratvars gcd = '$gcd algebraic = ,$algebraic))
(print
(ratfac = ,$ratfac))
(merror "Unable to find the spectrum")))

 (setq res ($fullratsimp (ncpower (sub (mult z ($ident n)) mat) 1) z))
+
+ (setq res ($fullratsimp ($invert (sub (mult z ($ident n)) mat) '$crering) z))
(setq m (length sp))
(dotimes (i m)
(setq zi (nth i sp))
~~~~~~