Dear Dov,
I don't think there is a relationship between the modules and your problem.
When the FTP layer is enable, Joomla use the parameters present in the "configuration.php" file when it needs to write into the files or create directories.
So the only relationship that could be in relation with the modules is that some modules needs to write on the disk and in this case needs to use the Joomla FTP layer.
I think you should better verify that both subdomain.slavedomain.com and
www.slavedomain.com correspond to the same website.
If they have different different slave sites for each domain the check both FTP parameters for each individual slave site.
And additional information that may be important and that I have explained in the previous reply is the path where you deploy the slave site. If the "main, root" directory is not identical to the one that joomla expect, in this case, it does not perform the replace with the FTP root and you can have JFTP error.
Setup of the FTP layer is not easy.