Troubleshooting
When you get an Error 403 while logging into Workplace or another application with your Volt account, you can try the following:
Log in to your Volt Account in Google
Go to https://gmail.com (on Android/iOS devices: with your default browser) and log into your Volt account there. Then try it again.
Log out of other accounts
Log out of other accounts to which your device is connected. Start with the Google accounts and then try it again.
On Android/iOS devices, start with logging out of the accounts you are logged into in your default browser, then try it again.
Try it with another browser
Install another browser, (maybe set it as default) and try it with that one. On Android/iOS devices, Firefox Focus/Firefox Klar (name depends on language) has yielded good results. After logging in, you can restore your former default browser setting and/or remove the additional browser again.
PC only: Use an incognito browsing window
Open an incognito/private browsing window in your browser and use that for logging in.
Explanations
This error occurs because you are trying to connect to a Volt application with a non-Volt account. In general this is a private Google account which is also connected to your device.