Why are hybrids infertile?
A critical step in meiosis is the formation of tetrads. In diploid organisms like donkeys, they have a paternal version and a maternal version of each chromosome. Two chromosomes 2, for instance. During prophase 1, these two "matching" or homologous chromosomes form a tetrad and cross over, swapping alelles. There is no recombination and no tetrad formation in mitosis, which is used for growth and daily living.


  How is the meiotic process disrupted?


In a hybrid, there are no matching maternal and paternal chromosomes. In the zebroid testes or ovary, a lonely donkey chromosome 2 is wandering around, looking for another donkey chromosome 2, and there isn't a homologous chromosome. This mucks up Prophase 1, and the first division pulls crazy numbers of chromosomes to each daughter cell, and the gametes from this division are likely full of extra chromosomes and missing some critical ones.


  Apparently this difference doesn't obstruct producing (infertile) offspring. How comes the process of recombination of such vastly
  different number of chromosomes in gametes is viable?


An embryo needs a solid set of genes to grow up, but chromosomes don't have to be homologous for mitosis, and organisms use mitosis for growth and living, not meiosis. As long as the crazy mis-matched chromosomes in a zebroid have everything needed for a healthy organism, the hybrid will be healthy. It isn't until the zebroid starts to make gametes via meiosis that a problem occurs, as the homologues ONLY form tetrads in making eggs and sperm. The inability to recombine only occurs in the making of gametes.


  54 is an even number. How comes zebroids can't just normally produce
  fertile offspring with other zebroids of the same number of
  chromosomes?


Because even chromosome number doesn't help if they are not each homologous with another chromosome.