Alex Pankratov :
Jul 04, 2021Error 206 stands for "The filename or extension is too long." 
When it comes to specifying paths, Bvckup2 uses extended format , which allows for paths of 32K characters long, so this error should not be reported. You will need to check server-side logs because it's basically a bug in its SMB implementation.
In terms of a workaround you can try and use explicit network path instead of a mapped drive .