[TYPO3-english] BE module Languages make exception 1437078780 (7.6.3)

[Robert Gonda]

TYPO3 Exception
#1437078780: Extension Manager is in offline mode. No TER connection available.

TYPO3\CMS\Extensionmanager\Exception\ExtensionManagerException thrown in file
../typo3_src-7.6.3/typo3/sysext/extensionmanager/Classes/Utility/Repository/Helper.php in line 148.

TYPO3\CMS\Extensionmanager\Utility\Repository\Helper::fetchFile("https://repositories.typo3.org/mirrors.xml.gz", "...");

Any solution, please?
URL https://repositories.typo3.org/mirrors.xml.gz is available.