Hi,
We recommend using the -rlt options instead of -a which is a shortcut for --rlptgoD. The -a preserves permissions which can cause the infamous “disk quota exceeded” problem.
In regards to the problem you are reporting, it could be that you are transferring a large number of files and its taking a long time to generate the “incremental file list”. This Stack Overflow post deals with a similar issue. The resolution happened to be to use -rt instead of -a so it might be worth a try.