Randomly 403 HTTP Status from IIS

I have a ASP.NET (4.7.2) WebForms application who run from long time. Since some days, for one ASPX page (always the same), users randomly receive 403 status. I activate trace on IIS (10) and I see this detail :

MODULE_SET_RESPONSE_ERROR_STATUS - Warning ModuleName="ManagedPipelineHandler", Notification="EXECUTE_REQUEST_HANDLER", HttpStatus="403", HttpReason="Forbidden", HttpSubStatus="0", ErrorCode="The operation completed successfully. (0x0)", ConfigExceptionInfo=""

On the Microsoft documentation, the substatus 0 is not mentionned.

I don't know what to do next. The error occurs 3 or 4 time per day.

Any ideas ?

Edit 1

When I compare two IIS traces for the same page (status 403 and status 200), I see what in the 403 trave, the following ASP.NET page events are missing :

• PreRender
• SaveViewState
• Render To go further, I override in my page following events with log and call to base :
• OnPreRender
• OnPreRenderComplete
• OnSaveStateComplete
• OnUnoad And after another status 403, I see in my logs that avents OnPreRender, OnPreRenderComplete and OnSaveStateComplete are missing.

How can I go further to find why these events are not called ?

• OnUnload

Thank you for all ideas and response. Finally, the 403 was returned by our application (we use an opaque private framework...). The problem is now corrected.