iChain not always single signing?

[kurtz76]

I have this situation: accelerator set up for secure access (ACL correctly configured and working) in http. A user has noticed this strange behaviour twice (in different days): after inserting his credentials to access the protetected resource (he is member of the authorized group in the ACL) he's redirected to the correct page but logged in as another user. No form fill is set up, OLAC parameters are passed for cn,,ou,employeeType and l attributes. Certificates are valid. This normally doesn't happen and everything works as expected. Any hint on the reason of this issue? How to investigate and solve? I cannot think of a way to reproduce this...
iChain Version is 2.3.278
Thanks in advance