ironpython2: IronPython stops to work with module (json) after a while

Python 2.7.11 on .NET Core 3.1 hosted in Azure on service fabric. Initially everything works, after a while a certain library fails, namely json

{
    "ClassName": "System.MissingMemberException",
    "Message": "'module' object has no attribute 'dumps'",
    "Data": null,
    "InnerException": null,
    "HelpURL": null,
    "StackTraceString": "   at IronPython.Runtime.Operations.PythonOps.ObjectGetAttribute(CodeContext context, Object o, String name)\r\n   at IronPython.Runtime.PythonModule.__getattribute__(CodeContext context, String name)\r\n   at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)\r\n   at Microsoft.Scripting.Interpreter.FuncCallInstruction`5.Run(InterpretedFrame frame)\r\n   at Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)\r\n   at Microsoft.Scripting.Interpreter.LightLambda.Run2[T0,T1,TRet](T0 arg0, T1 arg1)\r\n   at IronPython.Compiler.PythonScriptCode.RunWorker(CodeContext ctx)\r\n   at Ic.RestMocker.Core.Scripting.ScriptSession.Execute(String script, Dictionary`2 scope) in I:\\agent\\_work\\817\\s\\Ic.RestMocker.Core\\Scripting\\ScriptSession.cs:line 93\r\n   at Ic.RestMocker.Core.Controllers.ScriptsController.Post() in I:\\agent\\_work\\817\\s\\Ic.RestMocker.Core\\Controllers\\ScriptsController.cs:line 38",
    "RemoteStackTraceString": null,
    "RemoteStackIndex": 0,
    "ExceptionMethod": null,
    "HResult": -2146233070,
    "Source": "IronPython",
    "WatsonBuckets": null,
    "MMClassName": null,
    "MMMemberName": null,
    "MMSignature": null
}

If I create a script like

import sys
sys.path

Output is

[
    "D:\\SvcFab\\_App\\RestMocker.ApplicationType_App66\\RestMocker.ApiPkg.Code.1.0.0.RestMocker_master_20201123.2\\pylib"
]

Which is the correct path to pylib