As described in our update documentation, some steps are required to correctly use a new Felgo version that is based on a newer Qt version. For example, Felgo 2.12.0 is based on Qt 5.9, whereas Felgo 2.11.0 is based on Qt 5.8. In this case the old build kits will no longer work and should be removed (see here).
In any case, I recommend to remove and re-install Felgo with the installer from our website to have a fresh and clean Felgo and Qt installation.
Let me know if this helps!
In case this solution does not work for you or you are facing a different problem, please reply with detailed information what is is going wrong exactly. I’m happy to help!