I have the same issue on Jenkins 1.509.2, maven 1.509.2 (obviously), credentials 1.6, and config-file-provider 2.6.1:

Aug 14, 2013 8:43:41 AM hudson.ExtensionFinder$GuiceFinder$4$1 get
WARNING: Failed to instantiate. Skipping this component
com.google.inject.ProvisionException: Guice provision errors:

1) Error injecting constructor, java.lang.AssertionError: Outer class class org.jenkinsci.plugins.configfiles.maven.security.MvnServerPassword of class org.jenkinsci.plugins.configfiles.maven.security.MvnServerPassword$DescriptorImpl is not assignable to interface com.cloudbees.plugins.credentials.Credentials. Perhaps wrong outer class?
at org.jenkinsci.plugins.configfiles.maven.security.MvnServerPassword$DescriptorImpl.<init>(MvnServerPassword.java:58)

1 error
at com.google.inject.internal.ProviderToInternalFactoryAdapter.get(ProviderToInternalFactoryAdapter.java:52)
at com.google.inject.Scopes$1$1.get(Scopes.java:59)
at hudson.ExtensionFinder$GuiceFinder$4$1.get(ExtensionFinder.java:422)
at com.google.inject.internal.InternalFactoryToProviderAdapter.get(InternalFactoryToProviderAdapter.java:41)
at com.google.inject.internal.InjectorImpl$3$1.call(InjectorImpl.java:965)
at com.google.inject.internal.InjectorImpl.callInContext(InjectorImpl.java:1011)
at com.google.inject.internal.InjectorImpl$3.get(InjectorImpl.java:961)
at hudson.ExtensionFinder$GuiceFinder._find(ExtensionFinder.java:391)
at hudson.ExtensionFinder$GuiceFinder.find(ExtensionFinder.java:382)
at hudson.ExtensionFinder._find(ExtensionFinder.java:151)
at hudson.ClassicPluginStrategy.findComponents(ClassicPluginStrategy.java:318)
at hudson.ExtensionList.load(ExtensionList.java:295)
at hudson.ExtensionList.ensureLoaded(ExtensionList.java:248)
at hudson.ExtensionList.iterator(ExtensionList.java:138)
at jenkins.model.Jenkins.getDescriptorByType(Jenkins.java:1176)
at hudson.plugins.copyartifact.BuildSelectorParameter.initAliases(BuildSelectorParameter.java:99)
at hudson.plugins.copyartifact.CopyArtifactPlugin.postInitialize(CopyArtifactPlugin.java:35)
at hudson.PluginManager$2$1$2.run(PluginManager.java:352)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:888)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)
Caused by: java.lang.AssertionError: Outer class class org.jenkinsci.plugins.configfiles.maven.security.MvnServerPassword of class org.jenkinsci.plugins.configfiles.maven.security.MvnServerPassword$DescriptorImpl is not assignable to interface com.cloudbees.plugins.credentials.Credentials. Perhaps wrong outer class?
at hudson.model.Descriptor.<init>(Descriptor.java:262)
at com.cloudbees.plugins.credentials.CredentialsDescriptor.<init>(CredentialsDescriptor.java:56)
at org.jenkinsci.plugins.configfiles.maven.security.MvnServerPassword$DescriptorImpl.<init>(MvnServerPassword.java:58)
at org.jenkinsci.plugins.configfiles.maven.security.MvnServerPassword$DescriptorImpl$$FastClassByGuice$$6542c798.newInstance(<generated>)
at com.google.inject.internal.cglib.reflect.$FastConstructor.newInstance(FastConstructor.java:40)
at com.google.inject.internal.DefaultConstructionProxyFactory$1.newInstance(DefaultConstructionProxyFactory.java:61)
at com.google.inject.internal.ConstructorInjector.provision(ConstructorInjector.java:108)
at com.google.inject.internal.ConstructorInjector.construct(ConstructorInjector.java:87)
at com.google.inject.internal.ConstructorBindingImpl$Factory.get(ConstructorBindingImpl.java:259)
at com.google.inject.internal.ProviderToInternalFactoryAdapter$1.call(ProviderToInternalFactoryAdapter.java:46)
at com.google.inject.internal.InjectorImpl.callInContext(InjectorImpl.java:1018)
at com.google.inject.internal.ProviderToInternalFactoryAdapter.get(ProviderToInternalFactoryAdapter.java:40)
... 25 more

Aug 14, 2013 8:43:41 AM jenkins.InitReactorRunner$1 onAttained
INFO: Started all plugins

@Bruce AFAIR you need to upgrade the config file provider plugin

2.6.1 is the latest config file provider plugin.

My http://site/pluginManager/installed sure claims that I have 2.6.1 installed.

I have the exact same issue on LTS 1.509.2 with config file provider 2.6.1 installed....

I'm running Jenkins 1.527 with Credentials 1.7.3 and Config File Provider 2.6.1 and get the same error. Looking at the source, the exception is expected (albeit unwanted of course):

The outer class of MvnServerPassword$DescriptorImpl (MvnServerPassword) doesn't implement (or extend from something that implements) the Credentials interface (it doesn't implement/extend anything). This may have been intentional, but since MvnServerPassword$DescriptorImpl extends CredentialsDescriptor, the outer class needs to be assignable to Credentials.

https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/model/Descriptor.java#L258 (CredentialsDescriptor extends Descriptor with a type parameter of Credentials)
https://github.com/jenkinsci/credentials-plugin/blob/master/src/main/java/com/cloudbees/plugins/credentials/CredentialsDescriptor.java#L37

With Jenkins 1.5.30, Credentials Plugin 1.7.6, Config File Provider Plugin 2.6.1 I still get this error and I can't edit any job. Does a workaround exist?

This error also causes the Ivy plugin to fail because that has a dependency on this plugin to load.