public static class ExceptionExtension
        {
            public static string ToFullBlownString(this System.Exception e, int level = int.MaxValue)
            {
                var sb = new StringBuilder();
                var exception = e;
                var counter = 1;
                while (exception != null && counter <= level)
                { 
                  sb.AppendLine($"{counter}-> Level: {counter}");
                  sb.AppendLine($"{counter}-> Message: {exception.Message}");
                  sb.AppendLine($"{counter}-> Source: {exception.Source}");
                  sb.AppendLine($"{counter}-> Target Site: {exception.TargetSite}");
                  sb.AppendLine($"{counter}-> Stack Trace: {exception.StackTrace}");

                  exception = exception.InnerException;
                  counter  ;
                }

            return sb.ToString();
            }
        }

The following works for me:

       program-id. Program1 as "testexcept.Program1".
       procedure division.

           try
              call "nothere"
           catch myex as type Exception
              display myex::ToFullBlownString
           end-try
           display "after exception"
           goback.
           
       end program Program1.

       class-id ExceptionExtension static.
       method-id ToFullBlownString extension (e as type System.Exception, level as binary-long = type Int32::MaxValue)
                                              returning result as string.
       procedure division.
           declare sb = new type StringBuilder
           declare #exception = e
           declare counter = 1
           perform until #exception = null or counter > level
             invoke sb::AppendLine(counter::ToString & "-> Level: " & counter::ToString)
             invoke sb::AppendLine(counter::ToString & "-> Message: " & #exception::Message)
             invoke sb::AppendLine(counter::ToString & "-> Source: " & #exception::Source)
             invoke sb::AppendLine(counter::ToString & "-> Target Site: " & #exception::TargetSite)
             invoke sb::AppendLine(counter::ToString & "-> Stack Trace: " & #exception::StackTrace)
             set #exception = #exception::InnerException
             set counter = counter   1
           end-perform
           set result to sb::ToString
            
           goback.
       end method.
       
       end class.