no free buffers for rx

You can leave the fifo length to its maximum value, this memory is made with the FPGA's M9X blocks and isn't seen by the software.

The cause of this panic is that the TCP/IP stack doesn't have enough space in your packet memory to allocate all its buffers. Reducing the packet memory size actually made things worse.

You should put back the packet memory to 64k in SOPC builder and modify ipport.h to ask the stack to allocate less buffers, so that it sticks in your packet memory. If you have a look at your word document, it is described in step 3. The total number of bytes allocated by the stack is NUMBIGBUFS*BIGBUFSIZE+LILBUFSIZE*NUMLILBUFS, and this number must be less than the size of your packet memory. As you can't reduce BIGBUFSIZE the best option is to reduce the number of buffers, NUMBIGBUFS and NUMLILBUFS.