I used Plantuml more than gladly in Libreoffice until version 5.x. Now I'm at 18.104.22.168 and my old way of integration (installed by macro embedded in .odt file from Plantuml) does not work any more. Also there is a Plantuml - Libreoffice integration by lodahl at https://github.com/lodahl/Libo_PlantUML
which fails to get added to this release as extension. If by chance any of you faced this problem and solved it would you please let me know how did you manage it?