Execute SP On Linked Server?

I've created a stored procedure on several servers. All with the same name and all in a database with same names.
Now I�m trying to execute the stored procedures by executing a script.... I don�t get it to work!
I can execute it successfully by starting it manually:
exec [CIMPDB01CIMT1].zz_am.dbo.usp_SpaceMon
exec [CIMPDB01CIMT2].zz_am.dbo.usp_SpaceMon
and so on..... no problem

But when I try to execute it by using a script (and substitute the instance names with values taken from a table) it won�t:

declare @x int
declare @dbname varchar(500)
declare @SQL nvarchar(600)
set @x = 1

create table #databases
(ID int IDENTITY,name varchar(500))

insert #databases select instancelongname from instances where actief='J'

while @x <= (select max(id) from #databases)
begin
select @dbname = name from #databases where id = @x
print @dbname
select @SQL='exec ' + @dbname + '.zz_am.dbo.usp_SpaceMon'
print @SQL
execute @SQL

set @x = @x + 1
end

drop table #databases


Msg 203, Level 16, State 2, Line 17
The name 'exec [CIMPDB01CIM].zz_am.dbo.usp_SpaceMon' is not a valid identifier.

Please help....