Primavera 18 opens last opened project upon startup!?

Member for

16 years 3 months

do not exit Primavera by clicking the red X upper rigth 

ALWASY select file close all. 

Also as Santish said you may have to close or delete that last user session.

is this p6 web ?

Member for

20 years 6 months

You're going to need the exact wording of the error message, perhaps take a screencapture when it appears on screen (Press ALT+prtscn to capture just the active window). Take a note of the error and search for it on http://support.oracle.com

It may end up needing to delete out the last user session record to enable you to log in without a project open.