error 400, The server cannot process the request because it is malformed, It should not be retried

[Kenneth Foong]

I encountered error 400 when attempting to login to google workspace using a mobile debice whereby an SSO was already setup with 3rd party iDP.

The test sequence was conducted using 2 test users account (called it testuser1 and testuser2).
testuser1 was able to login successfully to the iDP. Subsequently, testuser1 account was removed from the mobile device.
Next, testuser2 was used to login. When the login prompted was by Google workspace, testuser2 account detail was entered.
The error 400 was immediately displayed "error 400, The server cannot process the request because it is malformd. It should not be retried"

Since testuser1 was able to login earlier, it means the 3rd iDP configuration is correct.
I am suspecting somehow when testuser1 account was removed, it may have affected testuser2 login by affecting the http request string that was sent to iDP.

Any suggestions what could be the cause.