Cannot backup Time Machine to Synology NAS due to "... problem with username or password."
Remove the drive as your backup disk in Time Machine, then open your terminal and run. beware that running this command will remove any other existing time machine destination disks! 

sudo tmutil setdestination smb://uruser:urpass@NAS_IPADDRESS/TimeMachineName


If you don't want to remove all the existing destinations, you can pass the -a argument.

sudo tmutil setdestination -a smb://uruser:urpass@NAS_IPADDRESS/TimeMachineName


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