Thank you very much for Comodo, it is one of the best free firewalls available.
My problem is the following:
When I try an FTP Upload with my totalcommander (wincommander), then sometimes the files have 0 bytes when they arrive at the server. It does not happen all the time though, but the ratio is at 30%. Even for the same file, the upload will work sometimes and sometimes it fails.
Your problem is documented on the Total Commander forums. Unfortunately, I can’t link to the posts directly, but this link will take you to the Search utility. Type: FTP Upload 0 bytes (or copy/paste the text in blue) as a search string and it will take you to the relevant docs.
I had the same problem. With the security level set to custom I could not upload even with Total Commander identified as an allowed app. When I set the security level to allow all, the upload succeeded.