Analyze a bit of javascript

There is a certain website I'm logging into a lot, but for some strange reason it rejects the username and password when there are auto-filled by my password manager. However, if only the username is auto-filled and the password is entered via copy-and-paste (from the password manager into the password field) then the login is successful. Your job will be to analyze the javascript on the page, see what may be causing this (e.g. some code that's triggered by onblur) and come up with a simple solution that I can run inside Chrome as a userscript, which will make the logins using the password manager succeed. E.g. if the problem is discovered to be with a missing onblur event, then the userscript will check every 500ms if the password field is filled and if so send a direct onblur event to the field and then submit the form.