Read this: https://ww.wp.xz.cn/support/topic/problem-after-upgrade-to-v6-4-1/
It worked for my problem 🙂
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This reply was modified 2 years, 11 months ago by eiserfeyer.
Thanks for your help. I downgraded to 6.3 and installed WP Rollback.
Hopefully the developers will find the bug in 6.4.1 🙂