In some sessions, i can't copy or edit any file from server

WinSCP 5.1.4 (Build 3020)
GUI Explorer
Windows 7 Home Basic Service Pack 1

Hello:

I been using WinSCP for a while and this is the first time this kind of problem appears to me.

I have 2 web servers, server A (FTP protocol) and server B (FTP protocol too).

In server A I have php, javascript and image files (the usual stuff). I can connect to the server, copy the files to my computer, replace modified files, etc. Everything is ok.

The problem is on server B that has the same stuff. I can connect to the server, but i can not copy the files to my computer, replace modified files, or even edit them using the text editor of WinSCP. Is strange because i was able to do that a few weeks ago. This happens only with php files. I am the owner of these files and have all the permissions on it.

Everytime i use "Edit" on a php file a Microsoft Security Essentials (MSE) pop-up appears and WinSCP give an alert that says 'Cannot open file "C:\...". Access denied'. If i try to copy the files in my computer the MSE pop-up appears and delete the files.

I don't know what is going on and what to do. The tricky part that in only happens in server B and not in A. Why is that? What i have to check?

Any solution woud be appreciated.

Thanks in advance.

Isn't the problem specific to a local path? I mean you may be copying files to a different local path from server B than you copy them to from server A.
Or maybe contents of the .php files, there may be some suspicious code in them.