This error is indeed a valid one, even in 1.4.6 version, uninstalling and re installing did not solve it, create a new app pool, with correct settings ie SolidcpServer with same password and user works, almost looks like microsoft buggers up the app pool name somewhere..

Well the error mentioned is pretty litteral, username or password not being valid.

assuming you verified the username is still 100% good --> it comes down to password.

This can be caused by anything even a simple password age policy (have to change password every x days)

Besides generic advice i don't think i can give you any more detailed scenario for this as it's really environmental dependent.