Do you see this message... 'Your cloud database is in synchronisation with another desktop database and cannot be edited directly with the desktop software. You can work in read only mode or change your cloud database to a direct connection for all users.'. This is shown when you have been synchronising your desktop database with a cloud database, then you try to open your cloud database directly with synchronising.
You must choose if you want to use a synchronised connection (local and cloud database) or a direct connection (cloud database only). You cannot use both methods, if you try to use both your data will become corrupt as the databases try to sync changes differently.
You can see the advantages and disadvantages of each in this FAQ
If you decide on the direct connection method and only want 1 database in the cloud please be sure you have no local databases open and delete (or rename) any old local databases you may have synchronised with your cloud database in the past.
If you have any local databases open, remove any settings in 'Tools/Options/Web/Web Sync' to prevent them trying to sync when you are connected directly as this will corrupt your data. With a direct connection no synchronisation is needed. Once you have removed all web sync settings you can then tell the cloud its not used in sync mode by visiting your cloud account with a web browser and clicking the Account > Settings page link, ticking the box 'Disconnect this online database from your synchronized desktop database' and click Disconnect.
You can now use it directly in full mode without seeing the message above.