The global inbox inside DEVONthink is ~/Library/Application Support/DEVONthink/Inbox.dtBase2 whereas the inbox folder in the Finder’s sidebar is ~/Library/Application Support/DEVONthink/Inbox. Now which one is missing?
Both are physically visible in the Finder. The global Inbox disappeared the the sidebar in DEVONthink itself. When I doubleclick on ~/Library/Application Support/DEVONthink/Inbox.dtBase2, DEVONthink starts up with the error message “dtBase2 seems to be in use.”. After clicking “Proceed”, DEVONthink opens up, but still: Global Inbox is missing in DEVONthink’s sidebar.
Thank you, @BLUEFROG. I have not compressed/deleted DEVONthink 3 before. After doing that and a reboot, all is fine now again, Global Inbox appeared again. I really should have followed the instructions (which I read, but not follow).