@ukandrzej
Yesterday I had same error
In Wordfence I have select 1st recommended action to fix site, but got down site (There has been a critical error on this website).
Today I fixed it by uploading last backup:
1) go to host get backup ( I used UpDraft plugin )
2) reinstall new WordPress (on same domain/ into same folder)
3) setup plugin and upload backup
All work for now
