I solved this by re-installing the app…
for some reason the desktop app created a bad redirect URL which originally was:
https://(someword)-(someword)-(somenumber).hooks.n8n.cloud/rest/oauth2-credential/callback
on re-install it changed to:
https://(longrandomnumber).hooks.n8n.cloud/rest/oauth2-credential/callback
and this fixed the issue.