Why can't connect to MS SQL server?

Question

System administration

I got the user MSSQLServer in AD, under it I run MS SQL server on a piece of iron in the domain, mixed authorization is installed. Through SA it is connected.
In AD I get the user: UserSQL - I want to be connected to the server under it.
I log in via SA and add the HOME\UserSQL user for the UserSQL login, make comparisons with the necessary databases (db_owner, public rights). Actually, that's all. In the permissions, there is a checkbox "SQL Connection" (and for some reason the line "SQL Connection" is written 2 times)
Then incomprehensible things begin:
I connect via Managment Studio on the MS SQL server itself
Authorization type: Active DIrectory - passwords
Login: HOME\UserSQL
Gives an error : SSL Provider, error 0 The certificate chain was issued by an untrusted CA.
Protocols are enabled: Named Pipes (so I understood that SQL would be found by name) and TCP / IP
Shared Memory is disabled.
I unchecked the encryption checkbox in the connection parameters.
If I set SQL Server authorization and log in the same login: User login error, 18456.
I have been reading the Internet for a week, and I still don’t understand how to overcome it.
Where to look?
From what suddenly MS SQL for the user the certificate requested? How to disable?