First I would find some baseline, like last successful build. It seems that it is this one. I would re-run it (RE-BUILD COMMIT button in UI) and ensure it still works. If yes, check next failed build and try to understand the difference made by respective commit.
Another baseline can be the appveyor.yml in the open62541/open62541 repository, from which your one is forked and which builds OK on AppVeyor.