@Toxaris - to be fair,
(1) the discussions I had seen on this topic related to people who had installed the beta versions. The OP hasn't mentioned installing the beta, and nor had I on my machine, and I got this error as well
(2) the OP explicitly said he had uninstalled and reinstalled (a solution which did work for me)
(3) there is not one mention of it in John's release notes in the first post of the Sigil 0.5 thread.