Numbers cell format problem

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

If it works in the new account then boot back to the original account, then...

Are you running any VPN, Anti-Virus, or Cleaning apps?

We need to see what all is running, a report from this will not display any personal info...

Using EtreCheck - Apple Community

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info. Give it Full Disk Access.

https://www.etrecheck.com/

Give etrecheck Full Disk Access before running.

Thanks for Old Toad’s etrecheck instructions…

Slow iMac 2017 - Apple Community

In the non working account I'd try deleting these files, make sure Numbers is NOT runing...

~/Library/Containers/com.apple.iWork.Numbers/Data/Library/Preferences/com.apple.iWork.Numbers.plist

~ tilde indicates your home folder

Apple hid the Users' Library folders...

Method 1:

1 From the Finder, select the Go menu at top of the screen, and choose Go to Folder.

2 In the window that opens, enter ~/Library, and click Go.

Or…

 Use the Command-Shift-period

/private/var/db/BootCaches/[BigLongHexNumber]/app.com.apple.iWork.Numbers.playlist

Sorry, I'm at a loss of what it might be, but seems sure there'ssome corrupt file in that account.:(

Wild guesses, Numbers must be quit when deleting these...

~/Library/Containers/com.apple.iWork.Numbers/Data/Library/Caches/com.apple.iWork.Numbers

~/Library/Saved Application State/com.apple.iWork.Numbers.savedState