Just installed SketchUp 2025. I setup two custom toolbars and dragged the icons from other toolbars onto them. After closing SketchUp and reopening, the toolbars are blank. They’re still there, but no icons. This doesn’t happen in SU2024.
I cant reproduce this.
Perhaps your installation does not happened properly.
- Download a fresh installation file from here: Download All | SketchUp | SketchUp
(You should get this installer file: SketchUp-2025-0-571-242.exe) - Make sure no Sketchup is running, (The best woud be to restart your computer before next step)
- Important: Right click to the above mentioned installer file and chose Run as administrator
- Accept the UAC prompt (chose Yes).
- It may take some time while the installation window appearing. Wait patiently.
- Chose the install language and Click Next.
- Chose Repair
- Wait patiently until its finish and click Finish.
Check again if your problem still exist. 
_
Edited:
Well, I didn’t checked carefully enough. 
If you add 3rd party icons, that will be “lost” after restart. However icons from native Toolbars, will kept.
@colin ?
Thanks for the reply. I did “Run as Administrator” on the original installation. I went ahead and ran the repair option anyway, but I’m still getting the blank custom toolbars after restarting Sketchup. I’m also getting random crashes when opening Sketchup 2025. And, yes, all of the extensions I was using are 3rd party… I used the new extension migration tool if that matters.