Hi to all- A tech support engineer from Graphisoft, Istvan Moharos, was kind enough to get in touch with me and resolve this issue. For others who may run into this problem, here's a possible fix. It worked for me:
1. Stop the V18 Server module in the BIM Server Manager Web User Interface.
2. Delete the Mailboxes.db file- Default location: Applications/Graphisoft/BIM Server/BIMcloud Server 18/Mailboxes/Mailboxes.db
3. Start the V18 Server module n the BIM Server Manager Web User Interface.
After this the BIM Server will automatically create a new database file, and all previously sent messages will be lost (only the messages).