In order for the IHE XDS components to communicate with eachother it is important that they all use the same time. The XUA profile will use SAML tokens that have a limited lifetime. Therefore if the time of the servers running the components are not synchronised the SAML token will not be accepted.
IHE XDS describes this in their Consistent Time profile. (See http://wiki.ihe.net/index.php/Consistent_Time)
The easiest way to implement this is to make sure that your servers and devices use the NTP protocol to get the correct time from a time server.
For information on how to set this up in your environment please contact your system administrator.