unable to create new posts

After updating to 5.4 (was working yesterday on 5.3.2), I get this error every time I try to draft a new post: This block has encountered an error and cannot be previewed.

I’m doing the following:
1. Go to Posts->Add New
2. Enter a title
3. click in the empty first block
4. bam, error appears, no text can be added, and no blocks can be added/ deleted

I used the dev tools to make sure there was no caching in place, logged out and back in, nothing changed.
Disabled all plugins, still error.
Switched themes (Twenty Twenty), AHA, we have a working editor!

I’m using the Stargazer theme, which is no longer updated apparently, so no idea what to do next.

The JS Console has this:

TypeError: m.find is not a function
    at block-editor.min.js?ver=704b9a6d689b18b6982a8b2ae902e7ec:50
    at Array.reduce (<anonymous>)
    at block-editor.min.js?ver=704b9a6d689b18b6982a8b2ae902e7ec:50
    at Object.useMemo (react-dom.min.js?ver=16.9.0:205)
    at useMemo (react.min.js?ver=16.9.0:32)
    at Me (block-editor.min.js?ver=704b9a6d689b18b6982a8b2ae902e7ec:50)
    at block-library.min.js?ver=f710d3cc99e7a55a0d6a5418f3cfe33c:9
    at je (react-dom.min.js?ver=16.9.0:78)
    at ph (react-dom.min.js?ver=16.9.0:215)
    at lh (react-dom.min.js?ver=16.9.0:126)
ve @ react-dom.min.js?ver=16.9.0:103
f.componentDidCatch.c.callback @ react-dom.min.js?ver=16.9.0:114
yg @ react-dom.min.js?ver=16.9.0:62
xg @ react-dom.min.js?ver=16.9.0:61
Ti @ react-dom.min.js?ver=16.9.0:148
unstable_runWithPriority @ react.min.js?ver=16.9.0:26
Ma @ react-dom.min.js?ver=16.9.0:52
Ia @ react-dom.min.js?ver=16.9.0:140
ze @ react-dom.min.js?ver=16.9.0:118
(anonymous) @ react-dom.min.js?ver=16.9.0:53
unstable_runWithPriority @ react.min.js?ver=16.9.0:26
Ma @ react-dom.min.js?ver=16.9.0:52
mg @ react-dom.min.js?ver=16.9.0:52
V @ react-dom.min.js?ver=16.9.0:52
Be @ react-dom.min.js?ver=16.9.0:119
xi @ react-dom.min.js?ver=16.9.0:39