Exchange 2016/2019 coexistence. Certificate being used is a self-signed created on Exchange 2016, installed on 2019.
I've seen the error twice under normal operations and once when testing 2019 by taking 2016 off the network to make sure 2019 would work properly without 2016.
Arbitration mailboxes haven't been moved to 2019.
URL's on 2019 are the same as 2016.
Everything else working correctly besides this occasional error.
I found a couple of mentions of this error from a couple of years ago but none have offered a solution that has helped me.
Thanks