I have the same error, which also appeared suddenly when running the lumapi.FDTD() command. Error message: LumApiError: 'appOpen error: \n Failed to start messaging, check licenses...'
Until yesterday I have been running python scripts for Lumerical with my Python3 installation without any problem.
Regarding Lumerical I am running: Ansys Lumerical 2021 R2.3 Finite DIfference IDE.
My license are correctly installed (since I checked my License Manager and the Lumerical GUI opens without problem) and I added the line sys.path.append("/opt/lumerical/v212/api/python/") to my code. There were no updates to my OS (Centos7), nor were there updates for Lumerical. I also followed the steps in However, the error reappears.
Find attached a screenshot of my Python integration status window.