> (500) after 2 retries at link <some path> (from
> <some other path>)
Internal server error could be temporary or could be caused by overloading the
server via HTT. Change to one connection per second, one or two connections
and update.

> and Not Found" (404) at link <some path> (from <some
> other path>) errors
Bad links are bad links. Nothing can be done there except setting No Error
Pages so you know where the mirror ends.