What is the error number you are getting on the denied from the client? And what options are you using in your adskflex.opt file to handle the licensing pool?
I found the source of the license restriction.
Because Inventor license are not being properly allocated, I used the option file to restrict access to the Simulation licenses. The unintended user restriction was caused by case sensitivity in the username. The user was loggin in with capital letters. The option file default is for case sensitivity to be turned on. Once we turned it off, the problem was fixed.
This whole thing was caused by a license cascade that is not working properly, but the work around has us back in business.