I use autofs to mount a windows share on my router, this stopped working with “mount error(112): Host is down”.

Looks like it was due to some arch linux update, as per dmesg:

[ 5986.858242] No dialect specified on mount. Default has changed to
a more secure dialect, SMB3 (vers=3.0), from CIFS (SMB1). To use the
less secure SMB1 dialect to access old servers which do not support 
SMB3 specify vers=1.0 on mount. For somewhat newer servers such as 
Windows 7 try vers=2.1.

Specifying version 1.0 fixed it. (Note: this is just on my home network, you might want more security elsewhere).

Fixed autofs entry:

data -fstype=cifs,vers=1.0,username=me,password=notthis,uid=me,gid=users,workgroup=HOME ://myrouter/data