Errors in Syncing Leads to SF

Hi All, in the last few weeks we're seeing odd behavior of leads that are synced to SF. Seems like the leads are synced, but then get deleted with a weird exception (see below). If I try to manually sync the lead to SF after I while, I'm able to do so.

There wasn't any addition of validation rules or code to the SF side recently, or a permission issue (the integration user is defined as an admin)

Did anyone encountered a similar behavior?

Thanks

Itay