98% of the time the unknown error means, "I have run out of java memory and since I hit the ceiling I can't complete this action" and the unknown error appears and the model cannot be saved to the mart, or in other cases, opened from the mart, or other action.
The remedy is to increase the size of the Java memory on the mart server. Here is how to do that:
If running the Mart Server as a service (e.g. Start MartServer Service):
1: Stop Mart Server service
2: Modify the value registry key 'JvmMx' from 256 to 1536 on
HKEY_LOCAL_MACHINE\SOFTWARE\Apache Software Foundation\Procrun 2.0\MartServer\Parameters\Java
Windows 7 Enterprise:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Apache Software Foundation\Procrun 2.0\MartServer\Parameters\Java
NOTE: the above registry entry will not exist if the mart server service has not been created on the machine.
3: Click Menu 'ERwin Mart Server R9-> Start MartServer Service' to restart Mart Server as a service
NOTE: for further reference on Java memory settings please see