Watched folder error - "Exception while getting principal from Directory manager"

Hi! I've been having problems with a LiveCycle ES installation.
I've configured a watched folder that starts a process with an Office
Document, converts it to PDF and applies a Rights Management Policy.
It runs on behalf of an Active Directory user.
Sometimes it works flawlessly, but most of the time it fails, giving a
ridiculously long failure log with the longest trace stacks I've ever
seen in life... resumed:
=======================
ALC-DSC-600-000: com.adobe.idp.dsc.provider.service.scheduler.impl.SchedulerRuntimeException : Failure to invoke the job [watched_folder_endpoint_name]

Caused by: ALC-FEP-011-000: com.adobe.idp.dsc.service.file.impl.FileProviderRuntimeException: Failed to get the context on behalf of user [username], domain [company_domain] for watch folder [watched_folder_endpoint_name]

Caused by: | [com.adobe.idp.um.api.impl.AuthenticationManagerImpl] errorCode:16386 errorCodeHEX:0x4002 message:Exception while getting principal from Directory manager| [IDPLoggedException] errorCode:12801 errorCodeHEX:0x3201 message:Exception while getting principal from Directory manager
chainedException:javax.ejb.TransactionRolledbackLocalException: null;
CausedByException is:
nullchainedExceptionMessage:null; CausedByException is:
null chainedException
trace:javax.ejb.TransactionRolledbackLocalException: null;
CausedByException is:
null
=======================
I vaguely suspect is the server's clock going out of sync with the domain controller's clock, but I tried everything I knew of about it with no consistent results.

It's LiveCycle 8.0.1 SP2 installed on a Windows 2003 Server.
Manual install, JBoss Clustered configuration (the second node is actually turned off for the time being)
SQL Server 2005 as backend Database
Users on Active Directory on a Windows 2003 Server -which is a "copy" of the main domain controller.
Watched folder is in a mounted share of another Windows 2003 Server acting as fileserver.

Any clue will be greatly appreciated!!