We have this weird situation, We have Sales Insights but we uninstalled it 6 months ago. Now only those leads are failing to sync to SFDC who had interesting moments tied to it.
We do not have any active interesting moment campaign either. We are not able to update score from Marketo or see program status getting updated in SFDC.
SFDC Error: _updateSObjectsById failed for 1 Lead record(s) - INVALID_FIELD: No such column 'mkto_si__Last_Interesting_Moment_Date__c' on entity 'Lead'. If you are attempting to use a custom field, be sure to append the '__c' after the custom field name. Please reference your WSDL or the describe call for the appropriate names.
I have not seen this before. You should contact Support for assistance because uninstalling SI seems to have caused a mapping or sync error. You might have to stop the sync, uninstall the Marketo package, and then reinstall on SFDC side. Doing that has it's own issues too.
Another theory may be that the Marketo User has had permission changes and can't see the right fields anymore.