Nano 2.2.6 is too old for my liking ...
Current's nano is 2.3.2.
I tried nano 2.3.1 from Slackware 14.0 : same problem.
The file is saved ok however!
Libmagic is from the file package.
Current's file version is 5.13.
I replaced /usr/lib64/libmagic.{la,so.1.0.0} by those from Slackware 14.0's file 5.11 : problem gone for ^X, ^O displays garbage (but saves the file ok).
Edit: Replacing /usr/lib64/libmagic.{la,so.1.0.0} did not resolve the problem after all. But downgrading the complete file package to 5.11 (as hinted to by allend below) did.