Math mode of plantuml

Hi all,
Having a self-hosted gitlab, I have installed plantuml recently. All diagrams seem to work, except math diagrams. I have followed this guide:

https://docs.gitlab.com/ee/administration/integration/plantuml.html

It makes sense that JLatexMath doesn’t work - I didn’t install it yet - but also AsciiMath doesn’t work, although it should be integrated, according to http://plantuml.com/ascii-math .

How can I start debug/solve this?