This is generally caused by the user's account password expiring on Windows 10 preventing Splashtop login. We can check this by following the below steps:
Find the unit on Unity and open a terminal session by clicking "Start" as shown in Image 1.
A correctly open terminal session is shown in Image 2
Enter the command "net user USERNAME" (replacing USERNAME with the account in question) and press <Enter>, as shown in Image 3
A successful execution of the command is shown in Image 4, and as can be seen from this example, the user uniled's account is active, will not expire, but the password expired on 03/05/2019 19:18:46 which will prevent the user from logging in to the unit.
Enter the command "wmic UserAccount where Name='INSERT USERNAME' set PasswordExpires=False" (replacing INSERT USERNAME with the account in question) and press <Enter>, as shown in Image 5.
A successful execution of the command is shown in Image 6.
Re-run the "net user USERNAME" command from step 3 to verify the account's password has been set to not expire, the result should now have change the "Password expires" line to "Never" as shown in Image 7.
The user should now be able to log on to the unit in question using their credentials.