When attempting to open in Citavi 6 a cloud project, you get the following error message:
A connection to the Citavi Cloud could not be established. The project could not be opened.
Solution
Log out of https://citaviweb.citavi.com and then log in again. Then, log out of the welcome screen in Citavi 6 and in again.
If you are still working with Citavi 6.4, make sure not to use the option "My university". Instead use the email address and password associated with your Citavi Account.
If you no longer remember your password, click "Forgot your password?" to reset it.
To log in, follow the steps in the FAQ article "Missing cloud project" under solution 1.