There is a new configuration option in SQL Server 2017 called CLR strict security and it is enabled by default.
Essentially when enabled, it treats SAFE and EXTERNAL_ACCESS assemblies as if they were marked UNSAFE and any assemblies that are not signed will fail to load.
Microsoft recommends that all assemblies be signed by a certificate or asymmetric key with a corresponding login that has been granted UNSAFE ASSEMBLY permission in the master database.
-- to disable EXEC sys.sp_configure N'show advanced options', N'1'; RECONFIGURE WITH OVERRIDE; EXEC sys.sp_configure N'clr strict security', N'0'; RECONFIGURE WITH OVERRIDE; EXEC sys.sp_configure N'show advanced options', N'0'; RECONFIGURE WITH OVERRIDE; GO -- to re-enable EXEC sys.sp_configure N'show advanced options', N'1'; RECONFIGURE WITH OVERRIDE; EXEC sys.sp_configure N'clr strict security', N'1'; RECONFIGURE WITH OVERRIDE; EXEC sys.sp_configure N'show advanced options', N'0'; RECONFIGURE WITH OVERRIDE; GO
Further information can be found here.