1. Firewall is blocking the new version of Malwarebytes. Open your firewall and allow Malwarebytes as a valid program.
2. Fix a possible prior virus infection of the PC. Copy rules.ref file from uninfected PC with Malwarebyes installed. In windows 7 this is located on the C drive at:
Install rules.ref by copying it to flash drive and installing it in the same location on the infected PC.
3. The Malwarebytes server down. Retry again later.
Check out what others say about this issue and review or add your own comments and solutions!