Forwarding a prot range is needed for PASV mode. The range you mantion seems a little narrow to me, but I don't know how important it is. Anyway you should forward the port the server is listening at and also the port number minus 1, in this case 26500 and 26499 (compare to port 21 and 20...), as this is how the first attempt to connect will work. Then add the needed range for PASV mode and make sure they match (settings in server and settings in router).
Hope it works...
(I have never used Serve-U, neither Linksys so the above is to be seen as a hint in general only.)
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unicorn
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