Evelin,
Is your Confluence instance also tied to another application, such as JIRA? The problem could be the one stated in this article:
https://confluence.atlassian.com/display/CONFKB/Logging+into+Another+Atlassian+Application+Logs+Me+Out+of+Confluence
If you continue to have issues, and the problem is replicable across browsers, I recommend creating a support ticket at https://support.atlassian.com and we can investigate for you.
Regards,
Shannon
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