This is a known bug in your version. We had the same issue in IS 4.2 SP5. The process often completes but the UI times out. If you go to the server, one of the IS logs provides the location of the zip file in the input repository. I think its the application service log. This will allow you to export in the short term.
Anyway, this is a known issue and was fixed in IS 4.2 SP5 patch 2. On SP5 patch 3, the performance issues with exporting have been resolved (from what I've tested).
regards
Adrian