Hi @Marek G,
Thank you for posting to Microsoft Community. Based on your description: Exchange mailbox cannot login on Outlook mobile devices with Android 8. I would like to cooperate with you to work on this case.
To clarify this issue, please confirm if “this version does not support Mobile Outlook” you mentioned means the Android 8 version does not support Mobile Outlook.
As you mentioned, only the Outlook Lite can work fine on this version. However, you cannot add the on-premises account into Outlook Lite It's a normal behavior since Outlook Lite do not support on-premises mailboxes, it supports Microsoft 365, and Microsoft Exchange Online accounts.
You could refer to Microsoft Outlook Introduces Outlook Lite for more information.
Therefore, you could migrate the on-premises mailbox to cloud which Outlook Lite supports. But we recommend you update the version of Android to support the Mobile Outlook.
Hope the information I offered could help, if there are anything I could do to help, please feel free to contact me.