Is there anything "uncommon" you did before it stopped working?
Mainly change of password to windows account, update of avast to newer version, turned synchronization on or off, enabled or disabled one touch login or it just stopped working without any "uncommon" activity?
I believe you are having problem but reports like this are very rare and we are not able reproduce it so more information I can get could give me a better chance to understand and possibly reproduce the problem so we can kill it off.
We also did a change in the past which ensures only one password storage per windows user. Due to this change it may happen that you have had old vault in your storage created with some password and by deleting current one the application automatically started loading this old one so you may be entering correct password but for different password storage.
Do you see forgot master password option in Avast->Settings->Passwords (in top of the frame)? If so you can delete password storage, create it again and get your data back through synchronization.
