Wow, I'm mildly astonished that even one Java run works on a 512 Mb machine, given what a memory hog Java is.
You could try the following:
- Find the line $config['jobe_max_users'] = 10; in <jobehome>/application/config/config.php. Change it to 1. Only one job submission at a time will then run.
- Find the line define('MAX_RETRIES', 5); in <jobehome>/application/libraries/LanguageTask.php and change the 5 to, say, 20. This will mean that jobe will wait up to 20 seconds for any currently running jobs to terminate before aborting with a "Server Overload" error.
I certainly don't guarantee that this will work, but it's worth a try. Your machine is seriously underpowered and I've never tested under these conditions.
If those patches don't solve the problems, drop me an email and I'll give you an API key to use our outward-facing jobe server until you can upgrade your own.