Or, for the non-advertising version of an answer, you could try shutting down Notes, opening a command prompt, changing to the Notes directory (e.g., c:\program files\ibm\lotus\notes), and running the fixup utility on the archive file (e.g., nfixup archive\mymailfile.nsf)

All about this method or others you can find at:-

http://www.filerepairforum.com/forum/oth...

If you are unable to fix the problem using inbuilt tools, then to repair Lotus Notes database, there is NSF Repair Kit available in the market which can help you to fix the issues in your database file.

You may test tool: http://www.nsf.repair/

Error: 'An error was encountered while opening a window' generally occurs when The Notes code is consistently unloading nnotesmm.exe, or Notes Notes becomes crashed, or some processes which are running in the background and cannot be started twice.

So, here is the link of two relevant guide which you can use for your help-

http://lotus-notes-recovery.blogspot.in/...

http://www-01.ibm.com/support/docview.ws...

When you try to open a window in Lotus Notes application an error message appears on the

screen. On every attempt while opening a window in Lotus Notes database, you will receive

same error message:

"An Error Occurred While Opening a Window."

After getting the above error message, you will not be able to open a window in Lotus Notes

application.