Jean-Frederic, which version of mod_cluster fix this problem?
1.2.5.Final according to the mod_cluster JIRA.
This is an old issue, (fixed in m_c 1.2.5, we have 1.2.8 in ER2) and I haven't seen it since. I can't reproduce it with EWS 2.1.0.ER2, switching to VERIFIED.