.net - Why is running `EXEC AS USER='xyz'` yielding "Unspecified error occurred on SQL Server." -

i finishing conversion of excel vba application excel document-level .net customization, , getting error

unspecified error occurred on sql server. connection may have been terminated server.

with hresult=0x80004005 , complusexceptioncode = 0xe0434352 running sql statement:

exec user='domain\userid' 

this construct used vet various test accounts, , appropriate permissions set on sql server side.

• the code runs fine within old vba environment.
• connection string in both environments identical:
  "provider=sqloledb.1;integrated security=sspi;persist security info=false;initial catalog=abc;data source=def;use procedure prepare=1;auto translate=true;packet size=4096;workstation id=qrst;use encryption data=false;tag column collation when possible=false;"
• the same connection string works fine other sql statements; exec user='xyz' fails (for acceptable values of xyz).
• adodb 2.7 being used in both cases
• the exception being thrown of type comexception no innerexception
• the exception source "microsoft oledb provider sql server".

does have thoughts on why error occurring? there perhaps trust issue involved using click-once deployment intranet network location? have run out of ideas tracking issue down.