Fixing security holes sometimes causes problems in perfectly normal activities. That’s the case if you’re a macOS user who relied on a specific kind of login method for Windows and Windows Server file sharing before June 2019. Microsoft released a fix for NTLM, a technology for managed authentication and security that dates back over a decade in its first version.
Two critical bugs in NTLM required updates for a seemingly every version of Windows and Windows Server released for several years that incorporate NTLM. These necessary updates broke Mac-based login sessions using standard Windows file-sharing protocols long supported in macOS.
If you’re in that boat and receive an error that there was a problem connecting to the server, you can follow Apple, Microsoft, and third-party advice to re-connect.
Read more at MacWorld.com