Issue with upgrading a XAR application with a conflict

I've done the following test in a 4.1 installation:

1/ Create an extension with a test page with a certain content by exporting a single page (with no history)
2/ Install that extension in a wiki
3/ Modify the page locally in the wiki
3/ Publish a new version of that extension by exporting a modified version of the page (with a conflicting modification of the same line)
4/ Upgrade that extension

The version is not upgraded but there is no error message. Finally I cannot install anything anymore as EM seems broken.

The following stacktrace overflow is seen:

at org.xwiki.logging.LogQueue.error(LogQueue.java:414)