First thing - Please read our Reporting Rules
After that, give us some more informations, about your PHP include (CGI, FPM...) and so on!
Please go back to Administrator User and check if there are some limitations for the reseller of your client. If yes, set the correct value there, and afterwards check if you can change the upload_max_filesize value for the customer.
I did understand you hint and solved the problem myself. First I have to change the parameters for the reseller then as the reseller I have to modify the client/domain. Very simple. Have not tested it yet though.