Hi Harm and welcome to the Community!
The OAuth2 connector in EasyMorph has a limited functionality because at this point connectors in EasyMorph are immutable (i.e. read-only). Therefore, the connector only works well with services that don’t expire their refresh tokens at all (e.g. Google), or expire them once in a few months (e.g. Microsoft). If a service expires refresh tokens quickly, as it seems to be the case with Exact Online, the connector won’t be able to update the refresh token stored in the connector settings and therefore will expire and require re-authorization.
All in all, It’s a known issue which will be addressed next year with the introduction of global mutable key-value storage in EasyMorph.