Solution by eNTen Juergen

You got this error because you installed the Project Server related
hotfix and
did not run the SQL script that gets updated. The SQL script is
WEBSPS.SQL
and should be located in the default directory that Project Server
installs to.

You will need to run this SQL script through Query Analyzer and run it
against the ProjectServer database. Make sure you select the
ProjectServer
database from the dropdown menu located on the top toolbar.

If you don't do that then you won't be able to log into Project Web
Access, or
Project Professional.