Relation does not exist problem

Hi,

I'm new to the list and hope I don't make too much mistakes ;)

I have a postgreSQL 8.1 here, where I have a little problem. Database is
owned by user A, I have a group g_a and a user B. Both A and B are in
group g_a. Using the grant assistant from pgadmin (up to 1.8b5) I gave
SELECT, INSERT, UPDATE, DELETE and EXECUTE rights to g_a for every
object in the scheme. Though when I try to select something from a table
with user B, I get the error "Relation _table_ does not exist"
(translated from german).
Any advice where I should check for the problem? I already googled of
course, and it's not the Upper/Lowercase problem...

Lars