I had the exact same issue with the terminal release of IP Communicator. I fired up Wireshark and got to the bottom of it. There is an extra 0x00 following the filename (or preceding the octet mode, depending on your point of view)
Just because I found it annoying, I got the source for tftpd-hpa, which Centos uses and have located the section where the change needs to be made. I am not a C programmer, but I think I can muddle through incrementing the pointer if this case occurs. If anyone is interested, the change needs to be made in the vicinity of line 1024 in tftpd.c
When I get it to work I'll post it.....somehow. I know how to cross compile for other target architectures, but I don't know how to package (.rpm, .deb,etc)
I am planning to name it in.tftp-cisco, so that the process can be easily identified and to make sure you running the right program when you try it out.
From my reading, this is an issue with post ver 7.1 IP Communicator. The changes I am making should not break compliant tftp transfers from compliant clients.