Give DEVONthink Full Disk Access
DEVONtechnologies' applications can interact with your system and other applications to create a interconnected computing experience. In Apple's parlance, "It just works". Or should I say, "It just worked"? To increase security in macOS Mojave, Apple has decided people need to give more permission to applications that want to access certain kinds of data on your machine. As of this writing, the protected areas are: Mail, Messages, Safari, Home, and Time Machine.
One of the things you need to do is enable DEVONthink in System Preferences > Security & Privacy > Privacy > Full Disk Access. This is especially important for DEVONthink Pro Office as it allows access for the Apple Mail plugin to operate correctly. If this option is not enabled, emails may import very slowly or not at all.
If you are not prompted to give permission to one of the applications, you can unlock the Security & Privacy > Privacy preference (noting you'll have to enter your administrative password to make the change), then press the plus button and browse to find the application. If the application is open, you will get a notice the access will not be granted until you relaunch it.