Currently all information about current downloads may be lost if xolox crashes while it is updating the resumeinfo.txt file. (It has happened to me twice).
One quite easy way of doing it much safer is:
Rename the old resumeinfo.txt to resumeinfo.bak
Write the new resumeinfo.txt in the same way as now.
Delete resumeinfo.bak

When starting xolox:
If there is an resumeinfo.bak, use it instead of resumeinfo.txt.


Additionally it would be nice if you flushed the temporary files to disk more often than now. My experience is that you only flush the temporary files when the connection to corresponding server is lost.
Flush a file in this context => close file then open file for appending. (and update resumeinfo.txt if needed)