TFTP failed - configuration file NOT FOUND

[ncb]

I'm playing to increase the speed of my modem (Haxorware 5101) by using differents MACs and Certs. When I try to use a MAC and Certs with speed 10/1, every things is fine but when I use a MAC and Certs with speed higher 20/2, modem cannot online with log like this:

0x000127d2 [CmDocsisIpThread] BcmCmDocsisIpThread::RestartTftpConfigFile: (CmDocsisIpThread) ERROR - Failed to TFTP config file after 4 tries! Aborting...
0x000127dc [CmDocsisCtlThread] BcmCmDocsisCtlThread::IpInitErrorEvent: (CmDocsisCtlThread) ERROR - IP helper returned kTftpInitFailed error! Restarting!
0x000127dc [CmDocsisCtlThread] BcmCmDocsisCtlThread::SyncRestartErrorEvent: (CmDocsisCtlThread) reason: 12 (kTftpInitFailed)
Logging event: TFTP failed - configuration file NOT FOUND

All these MACs and Certs are valide.