Site login blocked after fields filled

[I thought this only applied to Chrome, but I've now verified that it applies to IE as well.]

On some sites, Identity Safe will fill in the user name and password, but then the "login" button (or whatever the site has) is inactive. Other controls, such as a "remember me" checkbox, are still active; but the only way I can actually log into the site is to close the vault, first.

I haven't spotted any pattern. It only affects certain sites.

Chrome: Version 45.0.2454.101 m; Identity Safe: 1.0.5

IE: Version 11.0.10240.16431; Norton Toolbar: Version 2015.5.4.10