"The IP addresses are fixed" – Fixed in what way? Does each device takes its IP address without consulting DHCP? Or is the DHCP server configured to always give each the right addresses? Is there another device that could have hijacked 192.168.0.15 (inadvertently or not) due to its conflicting configuration or due to the DHCP server not really aware that this address shall never be given to other hosts? After a failed
ssh or sshfs attempt check if arp -n prints the hardware address (MAC) of your legitimate 0.15 in the line starting with 192.168.0.15.