"A constraint violation occurred" error when trying to set Exchange 2000 "assistant" field

"A constraint violation occurred" means "incorrectly formatted parameter". In this case you are probably trying to write a name to the "assistant" field while Exchange 2000 expects a Distinguished Name.

Example
Set assistant for three accounts to DN for account with full name "John Smith"

[Settings_Begin]
BatchSettings.Delimiter=TAB
SET AccountName=%col4%    // %AccountName% instead of %col4% below = easier to change column
BatchSettings.MarkerCol=7

SET ADDomainPath=dc=com/dc=acme/dc=d31
SET ADOU=MailOU
SET ADOUPath=%ADDomainPath%/ou=%ADOU%
[Settings_End]

[Machines_Begin]
\\MAIN04
[Machines_End]

[Batch_Begin]
SET AssistantDN=ADValue (FullName,John Smith,distinguishedName)   
MessageBox "%AssistantDN%"   
Data.Loop
 Exchange.2k.Mailbox.SetProperty %ADOUPath%/cn=%AccountName%,$assistant,%AssistantDN%   
Data.Endloop   
[Batch_End]

[Data_Begin]
         accname1
         accname2
         accname3
[Data_End]