Credentials error when searching

I have gotten this error at random times and on random workstations. 

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The provided credentials could not be determined to be valid. Please contact your system administrator for valid service credentials.(No Error Path)

Service URI: https://tessiturasvcs.grandtheater.org/test/ConstituentService/Constituents/Search?type=fluent&q=smtih&dup=true
Verb: GET
StatusCode: 401
StatusDescription: Unauthorized
Service Response:
[NULL]

ErrorMessages:
<ErrorMessages xmlns:i="http://www.w3.org/2001/XMLSchema-instance"><Errors><ErrorMessage><Code/><Description>The provided credentials could not be determined to be valid. Please contact your system administrator for valid service credentials.</Description><ErrorPath/></ErrorMessage></Errors></ErrorMessages>

XML Request: (None)

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Doesn't seem to be associated with the Tessitura login as I can login with that user on a different workstation and will not get the error.  The error persists across workstation reboots, manual application pool recycles and services site restarts.  On the workstation in question, the error appears in both live and test environments.  It will normally persist for about a day and then clear up.  I have reset the users Tessitura password will the same results.

Has anyone seen this before and have a solution?  Where are the credentials being passed from?