Thanks, Frank. Browsing the site itself gives no errors.
Testing leads to the conclusion that the problem is in the combination of Autoptimize and the W3 Total Cache Minify section. PageSpeed Insights can do its job when I uncheck the HTML minify settings in W3 Total Cache. The other option is to use Total Cache minify, but then I need to uncheck both HTML and JS options in Autoptimize. I am not sure yet which combination gives the best results.
The CSS sections can be combined without errors.
The solution in that thread does not work for my site.