I had the same problem that Gontard posted in
JENKINS-6420. The 'fix' that Teilo suggested took care of it; however, before finding that solution I took a look at the source code. I noticed that the m2Release plugin uses a deprecated method (getAbsoluteUrl()) in one of the classes (org.jvnet.hudson.plugins.m2release.M2ReleaseAction) and I wonder whether changing that would also solve the problem?
I emailed a suggested patch to Teilo, and he recommended that I make a change to the patch and submit it in Jira, so here it is.