have the same problem, seems to be a softwarebug (may be the reason is the special keyboard-driver used in banking-mode??). There seems to be no solution available at the moment. So I reinstalled my old Version (12.3.2280 build 12.3.3154.33) and banking-mode runs fine again.
Use the special avast removal tool (avastclear.exe) for deinstallation. Look for fragments of elder installations and delete them manually. Use an offline installation file (avast_internet_security_setup_offline.exe).
And after all that: deactivate the automatic updatefunction.
notice: of course that´s no final solution - an official bugfix is required.