Why is member server not able to find domain controler on its site, showing events 2084 and 2085 in event viewer?

This is raised at a point where I need to uninstall Exchange 2010 from a member server after migrating to a newer server. Uninstall fails with error "Could not find any available Global Catalog in forest". Searched the event viewer and found a huge quantity of event ID 2084 and 2085.

Topology :

• Site A, subnet 10.1.1.x, contains DC1 (Windows 2012 R2) and DC2 (Windows 2012 R2)
• Site B, subnet 10.2.1.x, contains DC4 (Windows 2019) and member server Mail2 (Windows 2012 R2)

AD is properly setup for each site, with adequate subnets, and correct DCs assigned to correct sites.

DC4 has IP 10.2.1.2, Mail2 has IP 10.2.1.10, same subnet.

On Mail2 server, I can successfully ping the DC4 server using its IP or FQDN, which says DNS resolution is working properly.

BUT, event viewer says "No active domain controler for site B, will use following off site domain controllers : DC1, DC2.

(Sorry if these are not the perfect english error messages, I have french Windows servers).

What could be the reason member server Mail2 cannot find DC4 as active server ?