I (re-)set up a little (working) mantis farm. Mantis version 1.2.19, Apache 2.4.10, PHP 5.6.3, Windows
Jenkins 1.599, Mantis-Plugin v0.26

This setup worked with my old Apache2.2 installation. After a new installation with Apache2.4 and updated components the connection from Jenkins to Mantis does not work anymore.

In Jenkins log I found this:

Feb 23, 2015 9:06:28 AM INFO hudson.plugins.mantis.soap.mantis120.MantisSessionImpl 
Mantis version is 1.2.X
Feb 23, 2015 9:06:28 AM WARNING hudson.plugins.mantis.MantisSite isConnect
Failed to connect http://<myserver>/mantisfarm/instance/ (cause: Error Type: SYSTEM NOTICE,
Error Description: Array to string conversion).

With Loggin in Mantis enabled in Jenkins-Config I clicked the "Verify" Button. The Mantis log says that my entered credentials are correct:

23.02.2015 11:27 CET ldap Found value '<connectionuser>@<here>.de' for field 'mail'.
23.02.2015 11:27 CET ldap User '<connectionuser>' authenticated

But the Jenkins config claims "Unable to login with the user name and password."

I don't know how to get more debug information to tear down this problem. JENKINS-23979 was closed by the reporter with no detailed information.