I created a timing diagram, which is correctly generated on the https://www.planttext.com/
Once integrated in my Doxygen documentation, I had a "Syntax error" message, related to some "FontSize" keyword.
I noticed that this definition was only in my plantuml.cfg file, which is defining all the common skins for my documentation.
If I copy/paste the content of this file in the https://www.planttext.com
editor, between the @startuml and the "robust" keywords, then I have the same "Syntax error" message also on the planttext website.
I also noticed the same behaviour in the past with @startditaa keyword (not working in Doxygen, certainly related with having a plantuml.cfg definings skins/scale configs, etc...).