The error pops-up in DS-User after providing the backup set credentials (new EWS backup set).
The Office 365 plugin is incompatible with the DS-Client version, for example the Windows DS-Client is v.126.96.36.199 (13.1 SP2) was already deployed and the Office 365 plugin v.188.8.131.52 (from Asigra v.13.1 ISO) was later on installed.
Re-apply SP2 for Windows DS-Client v.13.1 so that the Office 365 plugin gets updated (no need to uninstall DS-Client, just re-apply the service pack on top of existing deployment). If a hot fix was also aplied prior to installing the plugin (e.g. Hot Fix 1 for Windows DS-Client v.13.1 SP2), after re-applying the service pack, you must also re-apply the hot fix (DS-Client will actually try to re-apply it automatically if the hot fix file is still available on the DS-System).