To resolve this problem, uninstall and then reinstall the Endpoint Manager:
Uninstall the EPM:
Click on Module Admin. Select Endpoint Manager. Select Uninstall. In the upper right hand corner of the page, select “Process.” Click Confirm. A white box will appear on the screen. Normally, you’d see text showing the uninstall and get a button to return. Instead, you’ll get a blank white screen. Press the “BACKSPACE” KEY.
Reinstall the EPM:
Click on Module Admin. Select PBX Endpoint Manager. Click Install. In the upper right hand corner of the page, select “Process.” Click Confirm. Click Return. Click Apply Configuration Changes. Click Reload.
Now you should be able to add SIP Extensions without the blank screen problem.
Ok so the 1.8.1.3 distro does not install the end point manager on install anymore. You can install the end point manager from FreePBX Module Admin if you want to use it after install and it will work fine.