This error is bing spammed in RCON. i dont know the reason. I have tried restarting my server but still this error is showing up.

I had the same issue. I went check my player logs while happening, spaming as well that:

Server Exception: Mountable Cycle
(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 39)

NullReferenceException: Object reference not set to an instance of an object
at MLRS.TryGetAimingModule (Item& item) [0x00006] in <32dc9dfab36748728a319b6b3ccd8f41>:0
at MLRS.VehicleFixedUpdate () [0x00044] in <32dc9dfab36748728a319b6b3ccd8f41>:0
at BaseMountable.FixedUpdateCycle () [0x00039] in <32dc9dfab36748728a319b6b3ccd8f41>:0
at ServerMgr.FixedUpdate () [0x00018] in <32dc9dfab36748728a319b6b3ccd8f41>:0
UnityEngine.DebugLogHandler:Internal_LogException(Exception, Object)
UnityEngine.DebugLogHandler:LogException(Exception, Object)
UnityEngine.Logger:LogException(Exception, Object)
UnityEngine.Debug:LogException(Exception, Object)
ServerMgr:FixedUpdate()

I figure out that the problem came from the MLRS (genius I know). I went check it on my map, and I noticed I could not open the missile or control countainer. Then I did a ent kill on it, and the console stop spam the message... 😦

Anyway, check your player logs in AppData\LocalLow\Facepunch Studios LTD\Rust to get a clue.