I had a similar problem to @Eduardo's (and the previous post) with v1.10.0 on macOS.
I found the following error in the development tools log, that occurs when the plugin starts (go to the Help menu --> Toggle development tools and search for "plant").
joplin.plugins: Uncaught exception in plugin "com.github.marc0l92.joplin-plugin-plantUML": Error: ENOENT: no such file or directory, stat '/var/folders/z8/6_9tvj2d6931fmn5q64r1xjm0000gn/T/joplin-plantUml2-plugin/'
Once I created the missing folder and restarted Joplin, the rendering of diagrams was fixed.