Installed the latest WIP like every week but now my custom toolbar is empty.
Also updated Rhino 8 and the custom toolbar there is still there.
The same thing happened to me as well. 
I will of course look into this immediately, I’m so sorry for the issues caused and really appreciate the report.
YouTrack → RH-91628 RUIs and customizations get reset in the WIP
I’ve created a fix which I’ll get released ASAP.
The issue was to do with the rui file paths, they were wrong. It’s possible your customisations may not have been lost but Rhino simply looked in the wrong place for them
I got the same type of error.
– Half of my custom icons were gone. Managed to replace about 10 of them, even though the icon editor was crawling.
– Saved the RUI, shut R9 down and reopened — boom, now all icons and toolbars gone.
– Added a backup RUI: same problem. So I reverted to the previous release.
I’ve resolved this bug and it’s in a new release that just went out a few minutes ago. Please update to the latest and the bug should be solved.
If you didn’t do a reset, it’s possible your customizations are still in the folder and opening the latest “just works” but I’m not 100%.
If you did do a reset, you can check your %appdata% folder which keeps backups when you do that and your customizations may still in that backup.
I can provide more info if necessary.
Thanks. I installed the new build. Some of my buttons showed up again but the majority is gone. Recovered a *.rui file from two days ago. Problem solved for now.
I’m sorry for all the hassle