Hi Thomas,
thanks for getting back. Here are the links to Fluent Forms and resmush on the Wordpress depository.
- Fluentforms -> Fluent Forms
- ReSmush -> https://wordpress.org/plugins/resmushit-image-optimizer/
For Optimize Press, I have run a test. I deactivated Optimize Press Dashboard and Builder and had a look if the error is gone.
This is what I did:
- Created a staging website.
- Deactivated all plugins.
- Activated matomo, works fine then. No trouble without any other plugins running.
- Activated optimize press dashboard, still works fine.
- Activated optimize press builder, now matomo crashes. So from my understanding I’d say it’s a bug in combination with the Optimize Press Builder (v3).
Hope this helps, will send them a message regarding this issue and ask them to check for it as well. Have send them a support query and linked this thread, hope they will add some useful informations. 
Anything else I should do?
Sadly I can’t help with the CreateDefinitionHelper, as I don’t have server access. Sorry.