for FXP you really shouldn't have to open/forward ports(other than for LIST) since you aren't the one getting/transmitting data..
take a look at
http://www.dslreports.com/forum/linksys they might have more experience with your specific router and might know how and what to change to solve your FXP issues.
but also make sure that you are indeed using SSL...i don't think your router is advanced enough to be able to snoop on encrypted connections, so if it's messing with your ftp, i suspect you might not be connecting using SSL or using SSL but w/o encryption.