EDIT: Apparently an issue with updates from Windows (cred to Markuss): https://www.veritas.com/support/en_US/article.000116721 
Workaround below is still valid if you do not want to uninstall security updates.

Today was a fun day. The entire team (except me, being a consultant with your own computer ftw) started having issues reading data from our project's database. And with "issues" I mean "No connections working at all".

The exception they got was this:
"A transport-level error has occurred when receiving results from the server. (provider: Shared Memory Provider, error: 15 - Function not supported)"

No changes had been made to the connection strings, database layer code or anything related to this. We haven't yet found out what caused it but it's likely a change in AD policies or changes made to the antivirus policies. 

The workaround is to disabled shared memory connections. Instructions for SQL Server 2016 below (should be the same for other versions as well)

  1. Open "SQL Server 2016 Configuration Manager" (should be on the start menu)
  2. Select "SQL Server Network Configuration"
  3. Select "Protocols for MSSQL2016" ("MSSQL2016" being the name of the database instance)
  4. Double click on "Shared memory" and set "Enabled" to "No".
  5. Select "SQL Server Instances"
  6. Right click on "SQL Server (MSSQL2016)" and click on Restart (again, "MSSQL2016" being the name of the database instance)