These lines are repeated several times, first one at the time when remote ssh connection with the machine is lost (mac address is the address of eth used for Internet connection)
Mar 24 21:20:32 ubuntu kernel: [565820.188684] [UFW BLOCK] IN=eth1 OUT= MAC=54:e6:fc:81:b4:ce:f8:d1:11:7f:7b:de:08:00 SRC=126.96.36.199 DST=192.168.1.3 LEN=202 TOS=0x00 PREC=0x80 TTL=47 ID=58666 DF PROTO=TCP SPT=80 DPT=1158 WINDOW=1 RES=0x00 ACK PSH URGP=0 Mar 24 21:20:41 ubuntu kernel: [565829.310850] [UFW BLOCK] IN=eth1 OUT= MAC=01:00:5e:00:00:01:f8:d1:11:7f:7b:de:08:00 SRC=192.168.1.1DST=188.8.131.52 LEN=32 TOS=0x00 PREC=0xC0 TTL=1 ID=0 DF PROTO=2
SSH is allowed for all internal network 192.168.1.0/24 and for IP address from work. Writing this question and just lost again the connectivity with machine.
Any ideas? Nothing is changed to lead for this behaviour, just started a week ago ...