One can use GAP's PermutationCharacter() to avoid the troubles.

Currently we have a problem here:

sage: PG = PermutationGroup(["(1,2,3,4,5,6,7)","(5,6,7)"])
sage: PG.molien_series()
... weird error...

see the report in https://groups.google.com/d/msgid/sage-devel/42c6be87-614e-4a21-9273-91948f0c6216n%40googlegroups.com

CC: @tscrim

Component: group theory

Author: Dima Pasechnik

Branch/Commit: b7cf604

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/34854