issue receiving DMX512 protocol over ATmega88 USART
I don't think it is a good idea to call lots of functions from your interrupt handler. Try to change it so it only reads the data to a buffer, and clear any possible error. Then have on your main loop a function to check for received data and process accordingly. That would be a first step to debug your problem.

On a side note, have you tried sending other types of data, say, ascii chars through RS485 and see if the get to your board?