Synology NAS won’t authenticate users after v7 upgrade

Check the logs to verify the NAS is rejecting NTLMv1
Logon to your Synology NAS via Web browser
Main Menu (top left), Log center, look for Warning entries that reference NTLMv1

Option1 More Secure, More Hassle:
On each PC, start menu, type Local Security Policy, open it
Local Policy, Security Options, Network security: LAN Manager authentication level
In the Drop-down choose Send NTLMv2 responses Only

*This could mess with other, older SMB devices
If the File sharing server requests a LM or NTLMv1, your PC will still respond. There are more strict options to force your PC to NOT respond to those old protocols.

Option2 Less Secure, Less Hassle:
Logon to your Synology NAS via Web browser
Control Panel, File Services, SMB, Advanced, Others tab, Allow NTLMv1 authentication.

More info here
https://mariushosting.com/fix-synology-dsm-7-smb-clients-get-disconnected/