FYI: Because of update checker limitations, the plugin will offer to download
EPUBCheck v5.0.0-alpha-1.
Please change the following
EpubCheck.json setting to false, until the W3C releases the final version:
"github": false,
If the plugin has already downloaded the alpha version on your machine, please download
EPUBCheck 4.2.6 and replace the Java files in the Calibre plugin folder (....\calibre\plugins\EPUBCheck) with the EPUBCheck 4.2.6 files.
To display the Calibre plugin folder, select Preferences > Miscellaneous > Open Calibre configuration folder and double-click plugins.