Oh that was interesting. I pressed reload plugins and Friendica immediately stopped working, giving out no hint but server 500 errors.

Turned out that piwik plugin crashed on some random error (Uncaught Exception TypeError: "Argument 2 passed to piwik_analytics() must be of the type array, string given), but it suggests that somehow plugins were not used at all for a long time now. ?

I usually think php stuff as "immediately effective" since it always reloads and reparses everything. That was a refreshing blow not to trust even this.