There should be a more informative error message shown in the black console window at the bottom of the Arduino IDE window after you close Boards Manager. Post that here using code tags (</> button on the toolbar).
Click the link on the line after File > Preferences > More preferences can be edited directly in the file: - this should open a folder named something like Arduino15 (the exact name depends on your OS).
Delete all files in that folder except preferences.txt. Please be very careful when deleting files on your computer. When in doubt back up!
Open Boards Manager again to see if the download error still occurs.