Just to confirm, you enabled file transfers? (i.e. -config
api.filexfer=true )
Assuming you did that, start ZAP with:
-config api.incerrordetails=true
and see if you get any details in the error.
If not, enable DEBUG log for:
org.zaproxy.zap.extension.api.API
org.zaproxy.zap.extension.api.CoreAPI
https://www.zaproxy.org/faq/how-do-you-configure-zap-logging/
Best regards.