DisplayLink, Duet Display, Air Display, iDisplay, and iDisplay apps are affected by Apple’s latest OS update. Users of these apps reported that after installing Mac OS 10.13.4, their external monitor screens went black. App makers advising users not to update to the latest version of the OS if they wish to use external displays. It is unclear if this is an actual macOS bug or a permanent change that Apple made to its OS without letting app developers know in advance.
Source: https://www.bleepingcomputer.com/news/apple/latest-macos-update-breaks-support-for-many-external-monitors/