Offline.htm application disables the application but does not completely stop the compiler.
So, on the first DLL that you loaded, the compiler sees different ones and starts compiling again ... so when you try to execute the next dll, it is probably used by compilation and cannot be deleted / replaced.
There is also the possibility that even if you ask to disconnect the application, still follow the process and use this DLL.
What I do, I download all the files on the server in a different directory, I open app_offline.htm, and then wait for my whole pool to close (I have a message at the end of the application), and then I copy all the update files from the server quickly . Another thing is that with ftp, the load time is longer than the compiler start time to recompile global functions from the dll.
To see that even with app_offline.htm the system is still compiling, just use the Process Explorer and look at the compilation.
What is compiling? I think compile dll besides pages.
A message from the compiler trying to compile the program from the first DLL from which you downloaded the compiler starts and starts a new compilation, now you download the next one, and when you try to make compile, the dll is blocked by ftp.
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