Sorry, an uncorrectable error has occurred. I need help

I have a problem, a user will mark the following error and closes the lotus notes.

Sorry, an uncorrectable error has occurred lookup

handle: null handle

Press Enter to abort the application. Shows a red window with that

Need help to fix it. Thank you

Subject: Sorry, an uncorrectable error has occurred. I need help

are there any NSD files in the users data directory?

Subject: RE: Sorry, an uncorrectable error has occurred. I need help

If the files are there. But the problem happens very rarely. After a few days of normal use, the message reappears.Not what you might be.