The root of the problem is concurrency writing to the cache directory
files with same names.

to reproduce the problem use the standard example
draw.jnlp

#run javaws
WIN> javaws.exe http://matmech:8080/draw.jnlp
# quickly change modified time for draw.jar and draw.jnlp
matmech> touch draw.jar draw.jnlp
# run quickly javaws with the same url one more time
WIN>javaws.exe http://matmech:8080/draw.jnlp

If your http server do not generate Last-Modified field, or generate it incorrectly you can only run jnlp client two times.
Some times one of the running applications hangs up,
some times generate error message:

An error occurred while launching/running the application.

Title: Draw
Vendor: Sun Microsystems, Inc.
Category: Download Error

Unable to load resource: http://matmech:8080/draw.jar

java.io.IOException: rename failed in cache

at com.sun.javaws.cache.DiskCache.insertEntry(DiskCache.java:211)

at com.sun.javaws.cache.DiskCache.insertResource(DiskCache.java:197)

at com.sun.javaws.cache.DownloadProtocol$RetrieveAction.actionDownload(DownloadProtocol.java:463)

at com.sun.javaws.cache.DownloadProtocol.doDownload(DownloadProtocol.java:609)

at com.sun.javaws.cache.DownloadProtocol.getResource(DownloadProtocol.java:762)

at com.sun.javaws.LaunchDownload.downloadJarFiles(LaunchDownload.java:563)

at com.sun.javaws.LaunchDownload.downloadEagerorAll(LaunchDownload.java:469)

at com.sun.javaws.Launcher.downloadResources(Launcher.java:601)

at com.sun.javaws.Launcher.handleApplicationDesc(Launcher.java:257)

at com.sun.javaws.Launcher.handleLaunchFile(Launcher.java:151)

at com.sun.javaws.Launcher.<init>(Launcher.java:113)

at com.sun.javaws.Main.main(Main.java:153)