I installed the CU September 2020 (KB4484504 and KB4484505). After that, the command bar on libraries and lists and in the items grid are broken.
I created a new site in English language and there the command bar is visible. Is there any solution?
This is a bug introduced with this CU. Looks like this only happens to localized webs. It is already listed as known issue: https://support.microsoft.com/en-us/help/4484505/security-update-for-sharepoint-server-2019-sept-8-2020
Workaround is to use classic mode at the moment.
As far as I heard, there are already some cases open for this behavior.
Install KB 4484504. See the following documentation. This helped me resolve the missing command bar. https://support.microsoft.com/en-us/help/4461512/september-21-2020-update-for-sharepoint-server-2019-kb4461512