Event code: 4005

Hy Guys,
something weird is happening! is a couple of days that my users are experincing a "logging out" isses, the error message is:
Event code: 4005
Event message: Forms authentication failed for the request. Reason: The ticket supplied was invalid.
Here the data:
1. the website is running on 3 server behind a load balancer
2. yes, machine key is the same all across thw websites, because the configuration is shared and all servers are pointing to the same folder on a NAS, this is the key:
<machineKey decryption="AES" decryptionKey=" ... snipped for security reasons ... " validation="SHA1" validationKey=" ... snipped for security reasons ..." />
3. I created the keys using an console app as suggested here: http://msdn.microsoft.com/en-us/library/ff649308.aspx#paght000007_webfarmdeploymentconsiderations
4. the form auth config is
 &n ...