Thanks TIG!
My workspace is now saved after restart 
I did as you posted above with perhaps an additional step.
See below how I got it working, maybe it could help others.
First, read and follow TIG’s post. He is TIG!!! 
Fist some general information.
SketchUp pro version 18.
Windows version 10 with latest updates.
Floating toolbar problem was actually a ‘won’t save worskpace’ problem.
User account type on pc just in case, local account type (no Microsoft account linked to desktop or other).
1st try.
Logged into the only account on my pc which has administrative rights (maybe important)
Name of the account is not important but I’ll call it C001.
Following TIG steps ‘run as administrator’ from context menu with repair no luck.
Tried uninstall and install, updated shipped plugins, copied updated from appdata/sketchup18… plugins to programfiles/sketchup18…shipped plugins, no luck.
2nd try.
Looked at TIG post again, maybe he meant ‘normal user’ as in account with no administrative rights. (English is not my native language
)
Deleted Sketchup from the still only account C001 I had on my computer.
Created a new account C002, with no administrative rights.
Logged out of my current account and logged in to the new one.
Downloaded SketchUp, right click context menu ‘run as administrator’ to install SketchUp.
Pop-up appears to give an administrator-password for install because this is account hasn’t got it by default.
Started Sketchup and updated the shipped plugins as TIG suggested.
It to work!
3th, trying to get it to work on the first account as well.
This is where it get’s weird, for me.
Logged into the first account C001 and installed plugins, problem remains on this account. Logged into account C002 and still works fine.
Logged out C002 and logged into C001 account again and created a third account C003 with administrative rights so I could change C001 account to non administrative rights.
Now I could change the workspace on account C001 and save it.
Changed Account C001 back with administrative rights, added some more plugins with toolbars.
Now the plugins installed while not having administrative rights stayed in place but the plugins I just installed while have administrative rights float and won’t dock.
Again changed the account to non administrative rights, docked the floating toolbars to the top, saved the file,
opened it again and it worked!
Somehow the workspace is only saved when the desktop account has no administrative rights or when these rights are turned of.
Turning administrative rights back on doesn’t affect the already installed plugins and saved workspace.
For installing new plugins and save a new workspace administrative rights has to be turned of again or the account has to have no administrative rights at all 
with kind regards,
Cristian de Kok