DBA but cannot connect to SQL Instance - "User does not have permission to perform this action (Error 15427)"

I am a DBA with local sysadmin rights, and sysadmin rights on the SQL instance. Up until recently, i have been able to connect to a SQL Instance without issue. All i am trying to do is connect to a SQL Instance using SSMS. Nothing fancy here - just normal login with Windows Authentication. Although this also occurs if using SQL Authentication. Now i am getting an error message "User does not have permission to perform this action (Error 15427)". My colleagues whom are also DBA's receive this same error. And the default SQL 'sa' account also receives this error. I have never seen something like this before and am having difficulty finding helpful responses as they all seem to be focusing on the error number rather than the symptoms i am experiencing. I can confirm that all Protocols are enabled and ordered as per our environment, and i can confirm that there have been no changes to this SQL Instance (that i am aware of). I can confirm the SQL Services are running and that permissions for the account running it have not changed (it is a gMSA). The saving grace is that this is in our Test environment so no Production data is at risk here. So i also have a great deal of flexibility in things i can try to resolve. Is someone able to please help me figure out what is going on here? Happy to provide further details if required, SQL Server: 2016, Version: 13.0.6435.1, Edition: Developer, Product Level: SP3, Cheers! Update - further testing/checks We have a user who is NOT a DBA. They are in an AD group that has read/write on a couple of DB's. They are able to connect to this SQL Instance. If i use their same machine, i am unable to connect to this SQL Instance. I can connect using other SQL Logins that DO NOT have sysadmin. If i use a SQL Login that DOES have sysadmin, i cannot connect. It seems like any login with sysadmin privileges is unable to connect??