If you ever see that you can not login to Umbraco backoffice anymore even though you haven't changed anything, this could be because of 301 Url Tracker umbraco package . For my case; this package was messing up paths for Umbraco Backoffice API mehtods and deleting all records in icUrlTracker table has fixed my problem. There are other suggestions like updating the web.config useLegacyEncoding setting but this hasn't worked for me. Hope it helps.
Technical Architect | Lead Developer | Umbraco Certified Master | Umbraco MVP x 2 | Certified ScrumMaster
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