I had this issue as well, and was driving me crazy, where my mapped drives to my Synology NAS could not be accessed. The disk free space was not displaying, and it was coming up with Incorrect username or password when trying to access them. The only option
would be to disconnect and reconnect in every logon session. Deleting the entry in Credential Manager made no difference, leading me to believe it was not Windows causing the issue.
I know it won't help everyone, but for those with a Synology NAS, I eventually fixed it by going into Synology DSM and turning on SMB2 in the Control Panel > File Services > (under SMB) Advanced Settings > set Maximum SMB protocol to SMB2 and Minimum SMB
protocol to SMB1.
For the record, SMB 1.0/CIFS File Sharing Support was enabled in the Windows 10 features.