This issue is fixed in BIMcloud 2019.1. Install the update and make sure both BIMcloud Basic and all connecting Archicad clients are restarted after the Downgrade process.
In certain cases the BIMcloud Basic upgrades to BIMcloud automatically. Downgrade to BIMcloud Basic is not possible and an error message may appear.
Possible reasons based on the frequency of occurrence
The address of the BIMcloud Server has been configured to an IP address
Navigate to the Servers page on the BIMcloud Manager web interface. Select the BIMcloud Server and check the Primary address of the Connection settings. Make sure it is set to the default $protocol//$hostname:<portnumber of the BIMcloud Server> (E.g.: $protocol//$hostname:22001). After the Primary address is corrected try the downgrade process again.
User group exists on BIMcloud
Managing users in groups is a licensed feature and not available in BIMcloud Basic. Make sure you do not have any user groups or sub user groups before you start the downgrade process. If you still experience the above issue, please contact your GRAPHISOFT support representative. For the investigation BIMcloud log files might be necessary.