mount.cifs fails with mount error(2): No such file or directory

This is just a guess because I don't remember what the default settings were on Samba 3.6 but I think CIFS is trying to mount with an smb dialect that that version of samba didn't have yet.

Add the parameter vers=1.0 or maybe vers=2.0 to your list and see if you can connect:

mount -t cifs -o user=myuser,vers=1.0 //myserver/myshare /home/user/myserver/myshare/

EDIT: You may have to drop down the security level as well with another parameter: sec=ntlm

Try to install the package keyutils, it solved for me:

sudo apt-get install keyutils

(I've seen it here: https://unix.stackexchange.com/a/507464/472671)