Windows Live Custom Domains causes Entra account lockout

Hi everyone,

we have an on-prem AD connected with EntraConnect to EntraID since about 3 years. We only sync users and groups, no password hash or anything else. Since a few days 4 (out of about 250) users are constantly being locked out due to failed login attempts on an Application called "Windows Live Custom Domains". All 4 users are locked out not at the same time but within 30 min to an hour. This happens multiple times a day. As far as I was able to investigate Windows Live Custom Domains is a service no longer offered by MS or has been replaced with something else. How am I able to find out where this failed login attempts come from? If someone could point me in the right direction I would be very happy.

Thanks
Daniel