Hi @katjanurmi,
You can download the sourcecode of the precedent version of the plugin here : https://github.com/WebDevStudios/custom-post-type-ui/archive/refs/tags/1.12.1.zip
It comes from their official github repo.
I have just put the content of the archive in wp-content/plugins and deleted the new version of the plugin, and renamed the folder of the archive with the correct name (and then desactivated the autoupdate in backoffice).
It solved the problem for all my websites with this issue
