Hello!
Edit: MAKE BACKUPS incase it goes wrong. This worked for me, your server may be different.
Situation:
I play on a dedicated server hosted on Nitrado, this bug was present despite restarts etc.
I decided yesterday evening to check up on the files of the server related to the XUI configs that gets sent to the client when you join the server.
I found that the versions on my computer and the server weren't the same, some of them should have been updated with the new patch 20.4 B42 but the server for some reason did not.
Fix?
Upload the updated files from my PC to the server, restart and voilá.
Guide-ish:
Go to: "C:\Program Files (x86)\Steam\steamapps\common\7 Days To Die\Data\Config" (or where your local game is installed)
Go to: your server's "7 Days To Die\Data\Config" folder
Check the three folders named XUi, XUi_Common and XUi_Menu
Check the last edited time for the files in the above folders on both PC and server.
If any are newer on your PC than server that means your server also failed to update it's files.
For me, the following files were not updated on my server, but my PC had updated versions.
XUi/controls.xml
XUi/windows.xml
XUi/xui.xml
XUi_Menu/controls.xml
XUi_Menu/windows.xml
For safety, I renamed the server files and added the prefix "OLD" for backup in case it goes bleh.
Then I simply uploaded the above 5 files to their correct folder, started the server and it worked.
The issue in itself seemed to simply be a config file disparancy where your PC and server didn't have matching versions, which is why it has helped some to reinstall the server, thus possibly updating those files correctly.
Hope this helps,
Regards, Kevin.