I'm going to assume this has been fixed, since I've not seen it and I'm doing a LOT of testing on Python Platforms and Autodetect. Therefore resolving as fixed.

Please reopen ONLY if it occurs on a version of the plugin released after 17-May-2015, and provide a duplicable test case. Thank you.

I still encounter this problem. Here are the steps to reproduce:
1. Launch netbeans
2. Open a python project
3. Close netbeans without closing the project
4. Relaunch netbeans

You should get the error. The python project is correctly reopened. If I close the project before exiting the IDE I don't have the error.

Version:
Product Version: NetBeans IDE Dev (Build 20150527-e8112fc19595)
Java: 1.7.0_79; OpenJDK 64-Bit Server VM 24.79-b02
Runtime: OpenJDK Runtime Environment 1.7.0_79-b14
System: Linux version 3.16.7-21-desktop running on amd64; UTF-8; fr_FR (nb)


Stacktrace:
WARNING [org.netbeans.core.startup.InstalledFileLocatorImpl]: module org.jython in /home/jenselme/.netbeans/dev does not own jython-2.7.0-rc3 at org.netbeans.modules.python.api.PythonPlatformManager.getBundledPlatform(PythonPlatformManager.java:70)
SEVERE [org.openide.util.Exceptions]
java.lang.IllegalStateException: WARNING: Going from readAccess to writeAccess, see #10778: http://www.netbeans.org/issues/show_bug.cgi?id=10778 
[catch] at org.netbeans.modules.openide.util.DefaultMutexImplementation.enterImpl(DefaultMutexImplementation.java:455)
        at org.netbeans.modules.openide.util.DefaultMutexImplementation.enter(DefaultMutexImplementation.java:405)
        at org.netbeans.modules.openide.util.DefaultMutexImplementation.writeEnter(DefaultMutexImplementation.java:381)
        at org.netbeans.modules.openide.util.DefaultMutexImplementation.writeAccess(DefaultMutexImplementation.java:290)
        at org.openide.util.Mutex.writeAccess(Mutex.java:299)
        at org.netbeans.modules.python.api.PythonPlatformManager.storePlatform(PythonPlatformManager.java:217)
        at org.netbeans.modules.python.api.PythonPlatformManager.getBundledPlatform(PythonPlatformManager.java:98)
        at org.netbeans.modules.python.api.PythonPlatformManager.load(PythonPlatformManager.java:180)
        at org.netbeans.modules.python.api.PythonPlatformManager.<init>(PythonPlatformManager.java:59)
        at org.netbeans.modules.python.api.PythonPlatformManager.getInstance(PythonPlatformManager.java:112)
        at org.netbeans.modules.python.project2.PythonProject2.findProjectProperties(PythonProject2.java:208)
        at org.netbeans.modules.python.project2.PythonProject2.isProject(PythonProject2.java:251)
        at org.netbeans.modules.python.project2.PythonProjectFactory.isProject(PythonProjectFactory.java:29)
        at org.netbeans.modules.python.project2.PythonProjectFactory.loadProject(PythonProjectFactory.java:36)
        at org.netbeans.modules.projectapi.nb.NbProjectManager.createProject(NbProjectManager.java:401)
        at org.netbeans.modules.projectapi.nb.NbProjectManager.access$300(NbProjectManager.java:94)
        at org.netbeans.modules.projectapi.nb.NbProjectManager$2.run(NbProjectManager.java:314)
        at org.netbeans.modules.projectapi.nb.NbProjectManager$2.run(NbProjectManager.java:243)
        at org.netbeans.modules.openide.util.DefaultMutexImplementation.readAccess(DefaultMutexImplementation.java:252)
        at org.openide.util.Mutex.readAccess(Mutex.java:250)
        at org.netbeans.modules.projectapi.nb.NbProjectManager.findProject(NbProjectManager.java:243)
        at org.netbeans.api.project.ProjectManager.findProject(ProjectManager.java:164)
        at org.netbeans.modules.project.ui.OpenProjectList.URLs2Projects(OpenProjectList.java:1260)
        at org.netbeans.modules.project.ui.OpenProjectList.access$1400(OpenProjectList.java:137)
        at org.netbeans.modules.project.ui.OpenProjectList$LoadOpenProjects.loadOnBackground(OpenProjectList.java:474)
        at org.netbeans.modules.project.ui.OpenProjectList$LoadOpenProjects.run(OpenProjectList.java:394)
        at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1443)
        at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:68)
        at org.openide.util.lookup.Lookups.executeWith(Lookups.java:303)
        at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2058)

Reporters: Please include stacktraces as _attachments_. Thank you.

Notes: The anomaly only occurs with Setuptools projects.
I tried some root cause analysis, using Julien's test case. Results:

1. I believe Line 24 of python.project2 - RunSingleCommand.java should be deleted (as it was in the corresponding python.project)... delete this line:
PythonPlatformManager manager = PythonPlatformManager.getInstance();

2. Some conflict might (?) exist in PythonProject2.findProjectProperties.
That's about as far as I can take it. Reassigning to original author...

I was able to write a patch for this issue. You can get it here: https://bitbucket.org/Jenselme/nb-contrib/commits/branch/bz239131

The problem is that when loading the project properties or trying to auto-detect platforms in PythonPlatformManager, the project was trying to save the platform while only having read access to the file. I don't think it is necessary to save it here, so my patch basically remove the call to storePlatform. Furthermore, saving the platform doesn't work as explained here: https://netbeans.org/bugzilla/show_bug.cgi?id=252306

I wasn't able to figure out why it didn't happen when opening a project manually thought.

It would be great if I could get input on my patch quickly to finally close this issue.

Solved by http://hg.netbeans.org/main/contrib/rev/b047cef8879f