the is a form someone is trying to open and he gets this error. Others not. Master Data
Please post the complete error message.
If only one user is hitting the issue then most likely it's something to do with his/her browser and/or JRE settings.
Recommended Browsers for Oracle E-Business Suite Release 12 (Doc ID 389422.1)
How To View The Java Console And Enable Tracing For Sun JRE (Native Plug-in) (Doc ID 452853.1)