So, I got a bit further ...

1. using the Watch, I can confirm that the variable holds the correct clientID at each loop

2. tried to replace the "?" with @[User::clientID]

but got following error: Execute SQL Task: Executing the query ""exec SP101 'LIVE','" + @[User::Cl..." failed with the following error: "Incorrect syntax near '+'.".

3. last I wanted to use this variable as the source for the SQL statement in the Execute SQL Task, as you suggested, but unfortunatly I can't figure out how to do it

Still trying to find it out, but thanks a lot if you can help me ...