I discovered that I had this issue too, so I changed Line 19 in the config file to:
authentication.enabled = true
which allowed me to log into the demo account.
From there I went to People > Users and then I changed the password for my user.
Note the requirements for new passwords:
Begin with an alpha character
Be at least 8-characters long
Contain no blanks
Contain at least one upper case character
Contain at least one lower case character
Contain at least one numeric character
Contain at least one special character
My old password was not formatted this way, so maybe this had something to do with it.
Once I saved this change, I then modified Line 19 in my config file referenced above to
From there, I was then able to log in to SimpleInvoices using my username and the new password.
Note that there is no ‘Log Out’ link when logged in with no authentication. So what I did was open SimpleInvoices in a new *private* browser window to force me to login without use of the prior cookie.
Hope this helps!