Walls always falling
It would be great if you post your plugins, i am pretty sure it have something to do with your decay plugins and wrong config setup! 
Sorry dlmjlm
Here are the puglins used having a relationship with all that is construction.
AdminToggle - AutomatedEvents - AutoPurge - BuildingGrades - CompoundOptions - EnhancedHammer - IndestructableBuildings - NoDecay - RemoverTool - TimeOfDay - WipeSchedule - ZoneManager
can u Post now your NoDecay config?
{
"Debug": {
"outputMundane": false,
"outputToRcon": false
},
"Global": {
"cupboardCheckEntity": false,
"cupboardRange": 30.0,
"requireCupboard": false,
"usePermission": false
},
"Mutipliers": {
"armoredMultiplier": 0.0,
"baloonMultiplier": 0.0,
"barricadesMultiplier": 0.0,
"bbqMultiplier": 0.0,
"boatMultiplier": 0.0,
"boxMultiplier": 0.0,
"campfireMultiplier": 0.0,
"deployablesMultiplier": 0.0,
"furnaceMultiplier": 0.0,
"highStoneWallMultiplier": 0.0,
"highWoodWallMultiplier": 0.0,
"horseMultiplier": 0.0,
"minicopterMultiplier": 0.0,
"samMultiplier": 0.0,
"sedanMultiplier": 0.0,
"sheetMultiplier": 0.0,
"stoneMultiplier": 0.0,
"trapMultiplier": 0.0,
"twigMultiplier": 0.0,
"watchtowerMultiplier": 0.0,
"woodMultiplier": 0.0
}
}
your config looks ok, so for not using the plugin I can only assume that plugin is either not working properly or another plugin is causing a problem. I've seen your decay is everywhere 0 so try with the server setting decay.scale 0 that makes your decay off too.
It is because nodecay plugin by default does NOT cover twig structures and by looks of the pic you submitted the structure is indeed twig.

Check out the notes on the plugins page for full config options and setup.
Hello,
However : "twigMultiplier": 0.0,
I do not mind that the twigs are not taken into account also why so it happens with stone walls and metal ?

Merged post

Hi and thx,
I have tried several possible solutions including yours but nothing changes. I'm going to be crazy. So I thought maybe it came from the "ZoneManger" plugin. Here is its configuration (in the "data" folder)

{
"ZoneDefinitions": [
{
"Name": "pve",
"Radius": 5000.0,
"Radiation": 0.0,
"Comfort": 0.0,
"Temperature": 0.0,
"Location": "-26.58741 41.603 175.1834",
"Size": "0 0 0",
"Rotation": "0 0 0",
"Id": "83902852",
"EnterMessage": null,
"LeaveMessage": null,
"Permission": null,
"EjectSpawns": null,
"Enabled": true,
"Flags": "SleepGod, UnDestr, NoDecay, NoStability"
}
]
}

Merged post

Something weird happened. I deleted a car that was next to a player's house with the command "ent kill" and immediately, all the walls of his base were destroyed with. Would you have an explanation please?