To resolve the issue, install a DDEI 5.1 hotfix, with version number later than 1394:

1. Log on to DDEI web console via admin account.
2. Go to Administration > Product Updates then click Hot Fixes / Patches. The Install Hot Fix / Patch screen appears.
3. Click Browse and select the "ddei_51_lx_en_hfb1394.7z.tar" Hotfix file.
4. Click Install. Wait until the hotfix is installed successfully and the logon page appears.

5. Verify that the Hotfix has been installed successfully:

   1. Go to Administration > Product Updates then click Hot Fixes / Patches. In the History table, check if the Build is "1394" and Description is "Hotfix 1394".
   2. Under Help, Choose the About.
   3. Verify that the Hotfix number is "1394".
   4. Verify that the Hardware model is “Deep Discovery Email Inspector 7300 V1”.
6. Clear the web browser cache.