BIG PROBLEM:fatal 1:set user id user admin is not in eg shadow

We have postgres running on a linux machine
and we connect with 15 winnt 4.0 machines running ACCESS2000
When we change from access 97 to access 2000 we get every 15 minutes
following problem

after a process query
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fatal 1:set user id user admin is not in eg shadow

and also after we select
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waiting...

have sombody already have this problem on how to solve???

VERY URGENT !!!!