Marketo andbox to SF sandbox object sync

Hi,

I have connected a marketo sandbox instance to a SF full sandbox instance and the global sync seems to be working fine, all the leads / contacts are synced. However, when I try to perform the "Object Sync" process, it fails saying that details of the error have been sent to Marketo and nothing happens. Is there anything that needs to be adjusted for this process to work?

Thank you ahead for any input on this.