If I try to preview a previous revision of an entry, I get "an unknown error occurred"
This bug was due to an issue with the Stringify library and was fixed in Craft 3.3.16.1, as stated by Brad Bell in his comment on the original question.

Upgrading to Craft 3.3.16.1 fixes this issue.

(I'm posting this as an answer in case anybody else runs into the same issue.)