CIFS suddenly cannot connect [closed]

I have had the following CIFS mount in /etc/fstab set up to mount shares on a windows server for months:

//plex-server/d /mnt/plex_d     cifs    username=plex,domain=WORKGROUP,password=[REDACTED],uid=1000,gid=1000,dir_mode=0755,file_mode=0755    0       0

Suddenly this morning it won't connect. I tried restarting both machines. This is what dmesg is reporting:

[  568.071262] CIFS: Attempting to mount //plex-server/d[  568.071274] No dialect specified on mount. Default has changed to a more secure dialect, SMB2.1 or later (e.g. SMB3), from CIFS (SMB1). To use the less secure SMB1 dialect to access old servers which do not support SMB3 (or SMB2.1) specify vers=1.0 on mount.[  578.333520] CIFS VFS: Error connecting to socket. Aborting operation.[  578.333539] CIFS VFS: cifs_mount failed w/return code = -2

So then I do the reasonable thing and try each of the CIFS versions. For vers=1.0, vers=2.0, and vers=3.0 I get:

[  778.580177] CIFS: Attempting to mount //plex-server/d[  788.758030] CIFS VFS: Error connecting to socket. Aborting operation.[  788.758049] CIFS VFS: cifs_mount failed w/return code = -2

Please advise.