My database has been infected with very many serialized php arrays left by the All-in-One Event Calendar plugin from the WP plugin repository. The plugin was removed, its tables in mysql had to be removed manually in phapmyadmin, but nevertheless the plugin has left lots of traces in the WP options table in the mysql database. While I could remove some of the options, the plugin has injected serialized arrays in core functionality such as user roles (option_id 99) and cron (option_id 106). The plugin has installed a user role called "Event contributor" which cannot be removed. Somebody else asked for help in the forum to remove it from the database. The person got no useful advice and his problems were not solved but despite that someone has marked the unresolved topic as [resolved]:
Below is some of the mess in the
user_roles option id's.
in 99 'user_roles':
Other people have complained about the same issue, but apparently nothing has been done to resolve it, although the complaints were made a long time ago: