“HTTP redirect status code must be a redirection code, 3xx.” after udating to WordPress version 5.4

Not entirely sure, from which version I upgraded, but admin-site started to show error “HTTP redirect status code must be a redirection code, 3xx.” in the mid of update. Probably some module was queried about update instructions and there was some version incompatibility.

Solution
I moved temporarily all plugins from wp-content/plugins directory away, reloaded admin page (after which database upgrade started). If I got back access to admin site, I moved module directories back to their original location and started modules update. All fine now. Uh!