htmlpurifier v4.18.0 is causing errors in the log

[Grotax]

Original issue: #2883

Downgrade: #2924

Issue at htmlpurifier ezyang/htmlpurifier#428

This needs some investigation why this version causes errors, maybe our configuration is no longer working?

[blizzz]

I tried to repro the issue:

1. composer require ezyang/htmlpurifier which pulls in 4.18
2. add a feed
3. force-run the background job
4. check logs
5. repeat steps 2-5 a couple of times

Should I encounter the problem?

Also possible: dependency hell, by getting in conflict with shipped versions in other apps. Using php-scoper to integrate it into news own namespace could be a way, if this really is the case.