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.
It seems that switching to passive mode in Total Commander fixes the problem. It is not an explanation, but at least a workaround.