A search on "Failed to initialize database" brings up a whole load of threads. Mostly they report it is down to MSXML being needed. As far as I can see Adobe didn't fix the issue until RoboHelp 2015 and the thread below refers to the service pack.
In https://community.adobe.com/t5/robohelp/search-error-failed-to-initialize-database/m-p/3282842?page=... the user reports "I experienced the same error with the Search function not working in help compiled using RoboHelp 2015. I uninstalled and reinstalled RoboHelp, but that did not fix the problem. I was finally able to fix the problem by installing MSXML 4.0 Service Pack 2 and MSXML 4.0 Service Pack 3 Parser on my laptop"
It looks like you will be out of luck with RoboHelp 11 unless you want to try and find a later version of the MSXML service pack and risk how it works with your copy of RoboHelp.
Whatever you try, do backup first.
Fixed it! Found old installation files, re-installed, updated & now it's working fine. Why it went wrong is anybody's guess.